I usually start a clean slate and then let my experiences form my character. My last character was pretty righteous to begin with but then had to do some pretty terrible things that made him emotionally handicapped, betrayal, forced to murder innocents etc. My current character is an unscrupulous merc who've been doing anything for the right price, lying, stealing, cheating and murdering his way through Diamond City and Goodneighbour, for caps or chems, Cait being there to really get the worst out in him, but now it seems he's emotionally maturing as he learns more about her past struggles, and is forming a stronger bond with her (and with the bartender in the Dugout Inn, being of Russian descent like himself), this could be a stepping stone to feel involved in certain communities, or not, maybe it'll just be the two of them against the world. His eyes is set on the Institute and the fat contracts that must be offered there, perhaps he'll like the the place (probably not, though, too bright). All the psycho and buffout also takes its toll, especially after robbing those chem dealers and getting their entire stash (and killing his then-partner for his cut).
I played through the Mass Effect series once with the same character, an initially Paragon Shephard, but somewhere in the middle of ME2 it just made no sense for him to choose those altruistic, pacifistic, irrationally good dialogue choices when he had killed literally thousands upon thousands of humans and aliens, destroyed entire planets etc. he was bound to have become mostly detached from the suffering of others. Why save the council that has already betrayed you once, why spare the crying alien begging for her life when she has just jeopardized your entire mission, why trust this entire race that has in the past shown to be out of control, just shoot and ask questions later, life is cheap in the galactic scale of things.
