I'll be honest in saying that shes incredibly stupid. It took her so long to hear his side of things and now shes infected. Not much of a hero if she doesn't try to listen to a former hero trying to save a monster and his circumstances right off.
She probably figured Mai was someone he cared about who turned into a monster. Plus, it's not like she just charged him guns blazing, she isolated them in the ice dome and gave him a chance to surrender, he could have used that time to explain himself but he just tried to escape without saying anything.
can't blame him trying to escape when all the other heroes tried to kill her right off the bat. to him it was probably just a repeat of events when faced with her.
That's actually genius. The average person would see it as justice where the 100 hero subjugates the rogue hero. Only Sen knows that something is wrong.