This is for fellow Re-Animator fans, or anyone who's seen the movie enough times to know all the different little bits I'm talking about.
I've often wished the movie would've been a little clearer on precisely what led to Herbert to inject Dr. Gruber with the reagent at the beginning of the story. A deleted scene on the DVD has Herbert insist (when the topic comes up in conversation with Dan) that he was trying to save him, and judging by the way Herbert venomously admonishes Hill for stealing Gruber's ideas, I doubt very much Herbert intended for any harm to come to Gruber.
So, here's what I think. It's obvious from his brief appearance at the start that Dr. Gruber is not a young man. He actually appears to be quite old, in fact. So I think that over-work helping Herbert with the formula (or vice versa*) led to Gruber suffering a heart attack, and Herbert, for the dual purpose of wanting to save his teacher's life and to test the reagent on a human, injected Gruber with it. In fact, I'm willing to bet the two had some sort of pact wherein if one died the other would try the reagent out on them.
(*About precisely whose idea the reagent was, Hill says later, reading Herbert's notes, that it's "an extension of that old fool Gruber's work," so it seems to me that the idea for the reagent came originally from Gruber, and Herbert is continuing and perfecting his teacher's work.)
On a side note, I've often though about precisely what led Herbert to come to Miskatonic University of all possible places, where Gruber's old nemesis (and soon Herbert's) Hill teaches. It seems like a cosmic coincidence, of the kind that only happens in movies, but I don't think so. I think Herbert came there specifically because he knew Hill was there, and that it had everything to do with Gruber. See, we know Hill stole Gruber's ideas and got rich off them, so I think that, as a result, Gruber languished in obscurity and sought to prove himself through the reagent. Which, of course, led to him overworking and having a heart attack (or whatever).
So, I think, personally, that Herbert blamed Hill for Gruber's death, and came to Miskatonic for the specific purpose of getting revenge against him, first by exposing him for the fraud that he was and then by finishing Gruber's work on the reagent and rubbing his success in Hill's face. It's clear enough, at least, that Herbert's hatred for Hill stems from much more than Hill simply trying to steal the reagent idea from him. The sheer ferocity with which he kills Hill with the shovel suggests he's had bottled up hatred for this man for years.
Anyway, what's everyone else think? Agree? Disagree?