To be fair there's at least one trigger here, and it's right near the Joker's finger

Well, he is the villain. They are supposed to be wrong. Of course he triggers, since he brutally cripples and murders people. You are not supposed to find the things he does 'pleasant'.

The real question is how every other character reacts to this. The only solid argument I've seen with all this is that Barbara seems 'victim-y' in this. Just compare this to the Jason Todd murder (at least the version in 'Under the Red Hood', which is the one I am familiar with) and he is spitting his blood into Joker's face. The comments on the main article for this has a simple redesign with "angry eyes with 'you bastard' stare".