Up front I will admit that I'm primarily a RPer and PvE player, but when another ship gives me a reason to shoot at it I won't care (apart from a slightly different level of pucker factor) whether it's a CMDR or an NPC. In both cases my aim is to expeditiously reduce my target to vapor if I can or Sir Robin the heck out of there if I can't. I haven't been targeted much by players and teh vast majority of those have been pirates, who can be negotiated with. Only on about 4 times have I had players attack me with no coms and no valid reason (the 3 times I was attacked by another player when I was carrying a bounty are not on that list, they had a reason - not a very big reason to be true but still a reason) and who knows, they may have thought they had a valid reason to shoot at me. I'd certainly have opened fire on them if I thought I had a decent reason to do so, so I'm ok with them doing the same to me. If, to them, "because I can and it's fun" is a valid reason then fair enough. I've lost more of those encounters than I won but at least I left the odious little oik with a decent repair bill every time. Most of the time it was a decent and fun fight too - only once did it annoy me enough that I winged up and went looking for revenge. Never got it though, space is a big place and we never found the guy. Thems just the breaks sometimes.
Bottom line, in this game if to you there's a valid reason to shoot then pull the darn trigger.