Considering that probably 40%+ of everything in the Bungie universe (characters, stories, visual elements, ideas) was "borrowed" and/or modified from other sources (movies, TV shows, novels, short stories, arcade-era video games, etc.), they'd hardly have ground to stand upon in calling the Diablo guys to the carpet.
Nor do I think Bungie would ever complain about the Diablo guys.
Understand that I have a great deal of respect for the Bungie guys, and dearly love their work. They're "A1" in my book. However, they are and have been massive borrowers. Big time. The examples are far, far too numerous to dig into.
Understand that "borrowing" is very, very different from "stealing". Bungie are not thieves, at all. The stuff they've "added" to their world from other multitudinous sources has been modified just barely enough to squeak-by any possible claim of outright theft, and in fact the spirit in which Bungie operates in this "borrowing" regard is very positive.
Let's just call Bungie "The Masters of Homage," as they are like a dude who's running a D&D campaign and "integrates" cool ideas from his favorite movies and books into whatever campaign he's running. Bungie has done this almost as a rule, from day one. Thus, the Diablo guys are merely walking the same ground.
It would be cool to see some of the original Bungie guys in a modern (2012) interview. If asked or pressed about some of the more specific "borrowing" elements, it would be interesting to note their response, i.e., whether they'd discuss their inspirations and use of extensive "homage," or whether they'd categorically deny this "borrowing".
One wonders. I'm guessing the former, however, as they seem like great guys who've learned a healthy sense of humility and who've handled their awesome successes very well, indeed.
By the way, allow me to add to this that you cannot copyright or protect mere ideas and concepts (nor titles, for that matter)...only specific designs, characters, and so on. Also, the idea of an exploding bad guy is archetypical.
Having said the above, if Bungie pressed (and they wouldn't), in a court of law it might indeed be found that Diablo's creature is too close in concept, look, and execution to the Bungie "Wight" (not sure how Bungie even arrived as the name "Wight," considering it has no relation to the ancient historical reckoning of the name).
The legal result? If *anything* at all, on the extreme end, probably no more than a "cease and desist" order on future releases, or on the lesser end, maybe a requirement that future packaging include a "Thank You" to Bungie in some agreed-upon form.
Note also that I do understand that Bungie no longer has legal claim to the "Myth" universe, making the above theoreticals as moot as they can possibly be.
Which, elliptically, was the intention of the post to begin with.