I never attacked anyone first, but on the raids I used a key, I was prepared to. The simple lack of self-awareness of someone coming in and not immediately leaving would be enough for me to mow them down.
If I spend a key to open a locked door, I have claimed the room and everything in it. If you open the door, see me and come in anyway, you have become hostile.
One time I had a guy open the key'd door on Dam and sees me standing there with my barrel in his face and says -- "Oh, my bad!" He closed the door and ran away as fast as he could. Proper response.
However, a month back I had a particularly agonizing death. I spent a key on Spaceport and got a TON of great stuff. A guy came in so I fired a warning shot over his head and told him the room was taken. He took a few steps back, out of sight and never said anything. I waited with my sights trained on the door for more than a minute and never heard a sound so I assumed he fucked off. The moment I went back to looting I took a head shot and then a second to down me.
So, yeah. If I were still playing and you come in a locked room I opened, you're getting deleted. The people that double breach a room and complain about getting shot are just braindead faggots. Even if I choose not to be the aggressor in PvP doesn't mean that we're not competing for the same resources. My time is valuable and if your parents didn't teach you manners -- well, respawn it is.