If you're given an hour, is it bad to finish a job talk in half an hour?
Giving such a short job talk is a negative rather than a positive.  Generally the point of a job talk is to engage and impress the audience with (i) your research and (ii) you as a potential research colleague.  They want to hear a narrative that starts from a place they know and builds up to some impressive results, ideally that they would like to hear more about over the coming months and years.  If other candidates are "filling the space with good content" and you come up 33% (or whatever) short, then you risk creating the impression that you don't have 60 minutes worth of good things to tell them.

That's just an impression though.  People who care will look back through your dossier, so you needn't worry too much about that.  Moreover, usually there are only a small number of candidates brought in for job talks and interviews, and a talk / interview has to be really bad indeed in order for those who were supporting you (in the current job market, getting interviewed is certainly a very strong show of support) to change their mind.  Ending very short doesn't seem to qualify in my opinion: people who are not in your favor may point out that your talk was short, while those in your favor will think "So what?  S/he's still a good candidate..."

You shouldn't be thinking in terms of having killed your chances for a job but rather of having not fully exploited the opportunity you've been given.  Based on what you write I will guess that you are not very experienced giving 60 minute talks.  (And in fact that's how you're going to look more than anything: inexperienced at giving such talks.)  That's something that's easy to improve upon: practice more at giving such talks.  Nowadays most people use powerpoint/beamer, which makes it pretty easy to control the pace.  For something like a job talk, you should have at least one "dress rehearsal," i.e., a practice talk in front of real, live people who will stop to ask you questions.  If you did that and still came up that short, there's something to figure out: did you get so nervous that you skipped a lot of material?  If so, you can compensate by making more slides.

Let me end with an anecdote.  When I was on the job market, I gave one talk much earlier than the rest: it was from my perspective before the semester even started.  For this and various other reasons I did not have a "dress rehearsal."  I had the opposite problem: I had way too many slides (I mean actual slides, printed onto clear plastic and put on an overheard projector; this was 11 years ago) and I ended up plunking down slides full of dense text and then skipping most of the text.  I went on to give three more versions of the same talk at other places, all of which were much more polished.  Where did this shaky job talk take place?  At my current institution.  They hired me anyway, though I heard that one of my colleagues was motivated to look back at my teaching letters for reassurance.