For those of you trying to keep Dogmeat alive:
If I remember correctly, there’s a way in the military base to turn the forcefields to low power, allowing you to walk through them but taking a bit of damage. The trick is to leave them at full power, then trap Dogemeat so that he can’t move. This is most easily done with a decent Repair skill.
Of course, in Fallout EVERYONE can die.
Also, Tactics Ogre is absolutely chock full of avoidable deaths. Basically, the game lets you make either one or two big moral decisions, giving you three separate possible plot paths. Depending on which one you take, who lives, who dies, who joins you, and who even shows up at all differ tremendously. Further, there are a number of characters you have to give the right dialogue choices to or else they run off or get killed. Occasionally, you also have to bring an enemy down to critical health instead of killing them in order for the “good” outcome. There’s one particularly notable avoidable death in the fourth chapter:
You sister goes a little bit nutso. If you manage to successfully talk her down, she comes to her senses and joins you. If you fail, she kills herself.
Final Fantasy Tactics is worth mentioning for the ending:
If you think about it, if you refuse to let anyone join you, then they won’t die at the end. By turning them down you could save them.
Also, it may not be quite the same thing, but the old game Exile 3 had a lot of these, in a way. Basically, the world is beset by plagues of monsters. If you take too long to solve the problems, the world begins decaying and eventually the NPCs start dying off.