When Hamlet goes to see his mother (after the play), he hears someone lurking behind the arras (big tapestry hung on the wall in Gertrude's bed chamber). He stabs the blob and the blob slumps to the floor. It's Polonius, there to eavesdrop. Hamlet thought it was Claudius ("I took thee for thy better," i.e., of higher rank, namely, the king).
Rosenkrantz and Guildenstern were in on Claudius' plot to kill Hamlet—or have him killed. Hamlet discovered it, and in the best James Bond manner, switched letters on them, so they took the hit that was intended for him. They kinda asked for it.
Ophelia died primarily because she was pretty loosely wrapped. Can't blame that on Hamlet. Granted, his rejection was a bit harsh, but he did love her, and he was trying to keep her out of the mayhem that he knew was to follow.
In the cobbling of the fencing match, Claudius and Laertes replaced the foil that Laertes was to use with a foil (a foil is a blunt practice weapon) that had had the point sharpened, then added a lethal poison to that. In addition, just in case the well-practiced Hamlet should win all three hits, Claudius kept a cup of poisoned wine set aside, which he intended to offer Hamlet as refreshment. While they're resting for a moment between bouts, Laertes, who has already lost two of the three bouts ("A hit! A palpable hit!" and "A touch, a touch, I do confess!"), takes a swipe at Hamlet with the poisoned sword ("Have at thee, now!"). Hamlet, surprised, notes the scratch on his arm, then realizes that there's more afoot here than a fencing match. Laertes' weapon is sharp. He goes at Laertes, disarms him, and takes the sharp weapon himself. In the ensuing bout, he nails Laertes, whom, he now realizes, is trying to kill him in the guise of a friendly match. In the meantime, Gertrude, to Claudius' horror, chugalugs the cup of wine. The dying Laertes confesses to Hamlet what they were up to. Hamlet, knowing he's dying, figures "now or never" and skewers Claudius through the giblets. All fall down.
"All I did was kill a man who was fixin' to kill me."
Polonius was in the wrong place at the wrong time. Rosenkrantz and Guildenstern got caught in the trap they helped set for Hamlet. Ophelia came unglued and actually died accidentally. Gertrude also died accidentally, but that was the fault of Claudius and his little pack of Kool-Aid.. Hamlet killed Laertes basically in self-defense (after all, Laertes had tried to kill him), and then Claudius, who had pretty well established by that time that he had it comin'.
Suggestion:— rent or get from the library the 1940s Lawrence Olivier film of Hamlet. Ignore Olivier's voice-over at the beginning of the film ("This is a story about a man who couldn't make up his mind."), because Olivier doesn't actually play it that way. Although the play is somewhat condensed, (R. and G. don't actually figure), it's still the best movie rendition out there. You'll see all the elements I'm talking about. Great play!!