Got to say, it sucks to get jumped after you finish a mission. Some players exit the mission mode faster than others and take advantage of this time delay to whack other players exitig from this "net fog" . Kinda of like stealing candy from a baby, it is just too easy to get away with it so why not? After all GTA V is a game about lying, cheating, violent criminals with a short attention span and equally short temper, you expect that kind of play from online players. Protect yourself, I guess, just make sure you don't spawn near the mission's final destination unless necessary.
But,... here is the paradox, you need other players to co-operate with you in a mission in order to be successful. Killing your crew during a job doesn't make sense, they aren't the ones trying to whack your character so it improves your edge if you co-operate with other online players. At some point, trust has to be established between online players.
At the end of a recent online heist job in GTA V, I got whacked coming out of the netfog. Wasn't anything I could do. The leader of the heist was the one who whacked the other three,... we should've seen it coming but didn't. The heist leader kept trying to ram other players and jerking gunfire everywhere. Thought the player was just letting off steam, showing off, that sort of thing but no, the "bad actor" had other plans. We helped the heist leader achieve a nice chunk of change and the player rewarded us by ripping us off. That was a bad heist, a learning experience at best and it won't happen again. At the end of a job/heist/mission don't be anywhere near the final destination unless necessary. ;)
After that online incident I only did heists with other crew members, the first one went off really well. We used our mikes, we co-ordinated what we where doing and made 50K each for our efforts. Took us roughly 15 minutes to finish it.