Some people with anger problems do so because they themselves are being triggered. Help them deal with their past problem; compassion helps.
That’s good advice in some situations, but I don’t think it’s applicable in the situation they asked about. I think what you’re saying makes a lot of sense in situations in which you’re responsible for another person’s physical and emotional wellbeing. For instance, if you’re raising a kid, or working with a kid who has been through traumatic things, the first thing to keep in mind is that they’re doing things for reasons and that compassion goes a long way.
But you can’t have that relationship with every traumatized person you encounter. It’s not appropriate with a roommate.
And that person was asking specially about what to do about the fact that they are triggered by their roommate’s depression and anger. It was a question about how to make a living situation work, not a question about how to make a support relationship work.
Getting involved enough to help someone deal with their past problem is a completely different kind of relationship than they were asking about. And there’s no indication that either they or their roommate wants that.
And, when you are triggered by someone even at a relatively distant relationship, it’s generally not a good idea to establish an even closer relationship with that person.
Their roommate’s past is not their problem, and helping their roommate get over their past is not their responsibility.