I previously commented on this in another thread. It definitely struck me as strange that he just immediately runs off to grab a flamethrower to torch a woman without the context of her & her gang's murderous intent. As you said, she could have been a mentally ill woman having a manic episode. It also wasn't self defense either as he left out of harms way & returned just to kill her in a gratuitous, over the top fashion.
Any berserk person brandishing a firearm needs to be stopped as soon as possible by any means possible, including deadly force. Was she mentally ill? Oh so sad, but it doesn't matter; she was an immediate grave danger to anyone in the vicinity. Stop the threat, let God sort 'em out.