crashed: too many corpses being created

After installing a Yosemite update on my laptop, when it boots, it seems to all be fine until about the loading bar is maybe 2/3 done, then slows to a crawl and takes another 20 minutes or so to reach full - where it stays. I have no cursor or anything else during this time, but my caps lock button light will go on an off when I press it.

I've tried:
-Resetting PRAM/NVRAM
-Booting into recovery mode and repairing the drive/partition. This seemed to be done successfully.
-Booting hardware diagnostics and doing a check. It says everything is fine.
-Safe boot. Takes the same length of time and hangs at the same place, so honestly not sure if I'm even doing it right.
-Verbose mode says every single process has crashed, with 'too many corpses being created.'. (picture - note that it's not actually blurry, just scrolling pretty fast so a photo wasn't the best)

It's a Mid-2011 15" Macbook pro, 2.2GHz with a 750GB hard drive and 16GB of RAM. I'm still doing a few other checks (currently re-installing 10.10 using a thumb drive to see if that fixes it), but has anyone got any ideas or had a similar problem?