"There are definitely times this isn't true."

Sure, definitely. There are lots of fights with damage that hits absolutely everyone in the room, or NPCs that choose targets completely at random, or massive damage spikes, or computer problems that have nothing to do with character actions.

90 percent of the time though? Damage that hits everyone in the room won't one-shot them; it'll take them down to 80 percent or maybe 50 percent at worst and that will be it for the next minute. Healers will have plenty of time to top them off with HoTs and/or area of effect heals and almost any class can top themselves off as well. NPCs rarely choose targets completely at random, and if they do they can almost always be crowd-controlled or avoided. And massive damage spikes almost always hit the tank and/or are avoidable. As for computer problems, Internet connection issues happen to us all, but more often than not, problems can be mitigated by common-sense maintenance. (If you have 4 fps even at the lowest graphics settings, then do a virus scan, upgrade your hardware, or don't raid or do heroics. Sorry if that sounds elitist, but it's not like raiding is your salaried job.)

So 90 percent of the time, yes, actually, deaths of tanks are the healers' fault, deaths of healers are tanks' fault and deaths of dps are their own fault. For those remaining five percent, blame really weird, hard-to-prepare-for design in encounters, or Comcast.

Don't take this personally, though. People screw up! It happens! Own up to it, try not to do the same thing the next time, and it will all be OK. Be reasonable about it ("sorry, my fault", for example, if you stood in the fire too long, and run back without being asked), and reasonable people will be reasonable to you. "It's your own damn fault" may be true but doesn't mean "you're a bad person". (Well, not necessarily.)