If they were pissed when we ripped off one of their tentacles, imagine how angry they'll be now that we caught and killed one of them. And if you're not afraid yet, let me quote the article:

[Researcher Kubodera] also said that, judging by the number of whales that feed on them, there may be many more giant squid than previously thought.

They're Pissed, and there's lots of them.