Sorry to have not checked this in a week.
This is the page on personality traits in the wiki. Its "effects" are sadly only focused upon whether you get social skills, however...
Broadly speaking, the best defense against bad thoughts about seeing death or being in conflict is to just harden a soldier by getting them used to it until they "don't care about anything anymore". This can be done most safely by having them witness dead animals while also having things like a good dining hall to balance out their negative thoughts until they are inured to grisly sights. Nature as a trait (the "loves nature and animals" stuff) makes dwarves hate killing/butchering or caging animals, although I don't know if that relates to just
seeing dead animals, if you happen to have set up some traps that shredded some cave critters or wildlife that you could send dwarves on patrol to view. (Giving marksdwarves "live fire training" against caged creatures that are released into an exitless room surrounded by fortifications is a great way to both train marksdwarf and get them used to killing things, for example, and you can let down the drawbridge and let your melee recruits survey the damage on a patrol afterwards.)
Likewise, if you train dwarves up to being heroes (rank 11), they no longer complain about being on long patrols, and actually hate being taken off military duty. I think that makes all negative thoughts related to just being
in battle go away, although they still don't like being
hurt. Train a core military hard, and they'll get inured and you can send them out on perpetual patrol without worry, and then work on getting a rotation of militia in to start training up towards being hardcore. Until they hit hero, they're best only taking on single month of training at a time before being released back into the general dwarf population.
As always, the best bet is just padding your thoughts with happy thoughts from legendary dining halls and preferably whatever preference a dwarf has most. You might want to purposefully aim for a dwarf that has a preferred stone or something you have on-sight, so that you can make some doors in their personal room/barracks for them to get happy thoughts seeing.
Here's a
couple threads on general stress management.