Jinx’s actions are extreme, but I think the writers do a satisfactory job of setting up the final scenes. They show Jinx overhearing Silco and her misunderstanding that he will turn her in to show her feeling cornered, they show her losing previous allies like Vi and Ekko to emphasize how she feels completely abandoned, and they show her suffering hallucinations to demonstrate her fragile mental state. What she does is sad but completely tracks with her character and the setup from previous scenes. Then her attack on the council is (I interpreted) a means of lashing out after Silco dies, while also to get revenge and honor him in a way that she thinks would have made him proud. Again, completely tracks with the setup.