Li's unexplained actions

Can someone please explain the final episode ending

"So, are they going to explain why Li Tianchen went from chasing after his sister to instantly going to the stairwell to collect the phone and give the photo?"

The show did explain it. When Siwen's family stopped Li Tianchen and gave him the photo, he looked at the photo which had a drawing that Xixi made. It was a drawing of her and her brother. It said, I can't live without my good brother.

Now to explain all of this we have to go back into their childhood which showed Li Tianchen being a protective brother that cares about his twin sister.

Li Tianchen is a nihilistic character because his actions murdered his father (who he didn't care for) but also his actions murdered his mother (who he did care for). He was inspired (though it was hinted that it was infact manipulation) by Hat Boy (who was his mysterious friend) to stand up to his father and as a result ruined his life. Tianchen doesn't care about human life except for one person. His twin sister.

When Xixi runs away from him, it means that he lost her as well and that all the important things in his life are gone now. However, because he has her photo he can ask Cheng Xiaoxi to go back into the past and give him a new future (mentioned in Ep 10). Which is why he went to Lu Guang. However, what he didn't know is that Lu Guang won't allow Cheng Xiaoxi to change the past and simply relive it. So he wants to kidnap Cheng Xiaoxi and change the past to give him and his sister a better future.

"Also, why did his sister run out then of all times and not any other time?"

This was explained in Episode 9. In Xixi's fairytale (the one where she and her sibling are foxes), she blanks out and wakes up in the forest multiple times. Every time she blanks out, it was a show hinting to the viewers that she is being possessed by her brother. It's been established that when Li Tianchen possessses someone, they don't have any memory of being possessed and essentially they blank out.

So why are things different this time? Because we find out that Tianchen can loose control of a person if there is a powerful enough memory / emotion. When Tianchen uses Xixi as a tether to possesses Chen Bin, she gets flooded with his memories. His memories / emotions are that of a loving father which is something that Xixi didn't have. Xixi has guilt because her she told her mom to not divorce which led to her mother's death. That guilt also manifested after receiving Chen Bin's memory and wishes to live so he can see his daughter get married / have kids. This strong emotion break's Xixi from her brother's control and she runs away from him so that he can't use her.

Now why Cheng Bin and not other times Li Tianchen used her to murder other people. This was explained in Ep 9. The people that Li Tianchen murdered were the crows (aka people who spread rumors). While Xixi didn't like watching her brother turn into a hunter and hunting down the crows she kinda had no choice and had to go along with it since her adopted father is Qian Jin who represents the hunter that taught Tianchen how to hunt. And despite what her brother was doing, she still loved him.

However, the time the camel breaks its back and she has enough is when her brother stop hunting the crows (rumourmongers that ruined their life) and started hunting bunny rabbits (innocent people). The innocent people being Chen Bin.

@kawai_otaku In the first episode of season 2, it has been hinted that Cheng Xiaoxi was the one who was supposed to die and Lu Guang goes back in time to save him. This theory was all but confirmed in the final episode of season 2. What we the viewers are witnessing is a timeline where Cheng Xiaoxi lives and the events that unfold because he lives.

We then are introduced to Liu Min's brother (aka hat boy) who seems to know more about what is going on. So Li Tianchen joins forces with Liu Min's brother to change history (though in promotional art, it is implied that hat boy manipulated things so that Li Tianchen will join him).

So Season 1 was about Cheng Xiaoxi and Lu Guang living a happy life after Lu Guang goes into the past and changes history.
Season 2 is about Li Tianchen's backstory and how he ends up joining forces with Liu Min's brother (aka hat boy)

"So, are they going to explain why Li Tianchen went from chasing after his sister to instantly going to the stairwell to collect the phone and give the photo?"

The show did explain it. When Siwen's family stopped Li Tianchen and gave him the photo, he looked at the photo which had a drawing that Xixi made. It was a drawing of her and her brother. It said, I can't live without my good brother.

Now to explain all of this we have to go back into their childhood which showed Li Tianchen being a protective brother that cares about his twin sister.

Li Tianchen is a nihilistic character because his actions murdered his father (who he didn't care for) but also his actions murdered his mother (who he did care for). He was inspired (though it was hinted that it was infact manipulation) by Hat Boy (who was his mysterious friend) to stand up to his father and as a result ruined his life. Tianchen doesn't care about human life except for one person. His twin sister.

When Xixi runs away from him, it means that he lost her as well and that all the important things in his life are gone now. However, because he has her photo he can ask Cheng Xiaoxi to go back into the past and give him a new future (mentioned in Ep 10). Which is why he went to Lu Guang. However, what he didn't know is that Lu Guang won't allow Cheng Xiaoxi to change the past and simply relive it. So he wants to kidnap Cheng Xiaoxi and change the past to give him and his sister a better future.

"Also, why did his sister run out then of all times and not any other time?"

This was explained in Episode 9. In Xixi's fairytale (the one where she and her sibling are foxes), she blanks out and wakes up in the forest multiple times. Every time she blanks out, it was a show hinting to the viewers that she is being possessed by her brother. It's been established that when Li Tianchen possessses someone, they don't have any memory of being possessed and essentially they blank out.

So why are things different this time? Because we find out that Tianchen can loose control of a person if there is a powerful enough memory / emotion. When Tianchen uses Xixi as a tether to possesses Chen Bin, she gets flooded with his memories. His memories / emotions are that of a loving father which is something that Xixi didn't have. Xixi has guilt because her she told her mom to not divorce which led to her mother's death. That guilt also manifested after receiving Chen Bin's memory and wishes to live so he can see his daughter get married / have kids. This strong emotion break's Xixi from her brother's control and she runs away from him so that he can't use her.

Now why Cheng Bin and not other times Li Tianchen used her to murder other people. This was explained in Ep 9. The people that Li Tianchen murdered were the crows (aka people who spread rumors). While Xixi didn't like watching her brother turn into a hunter and hunting down the crows she kinda had no choice and had to go along with it since her adopted father is Qian Jin who represents the hunter that taught Tianchen how to hunt. And despite what her brother was doing, she still loved him.

However, the time the camel breaks its back and she has enough is when her brother stop hunting the crows (rumourmongers that ruined their life) and started hunting bunny rabbits (innocent people). The innocent people being Chen Bin.

So is xixi dead? I don’t think that was confirmed