If you're looking for feedback and thoughts about how one way one could approach this; these are mine.
If we were not that close, I would distance quite heavily from the relationship, if I kept him in my life at all. I personally would not do any further steps unless Liam were incredibly dear to me. Death and violence is a huge part of my life and it is not avoidable. It sounds like it is this way for you, too.
But if he was that important to me, and approaching this as if I were in your position:
I would be certain I would have to have a talk with Liam-- uncertain boundaries are a hard limit of mine.
I would mention that I need to have a conversation regarding clarity in his boundary towards violence, and ask him for a good time to meet up. This lets him adequately prepare for a discussion, which is important when the discussion will likely contain triggers.
I would ask if he'd like to know some of the questions beforehand to prepare his answers. I would keep the outline as snappy as possible without omitting the most important context.
Even if he didn't want the outline, I would keep it for myself for the discussion, so I could stay on task properly and not draw out a hard conversation.
I would talk to my therapist before reaching out, to feel more solid in my plan.
Other thoughts:
It's not a "wrong" thing to be traumatized by violence, but it does sound like his boundary might make your relationship incompatible. If he is triggered by what is a huge part of your life, there's only so much that can be done with that.
I do not think that your own feelings are invalid, and your concerns regarding this make a lot of sense to me. If it turns out that it would make your relationship extremely shallow, it's up to you to decide if it's worth keeping.