Just your run of the mill neighborhood creep being creepy. Still, I'll give him points for going the extra mile and putting his mouth on a stray.
It might have made for a good twist if it wasn't a run of the mill abyuse-fest. Pretty sure an artist tried something like that but never finished the pool.
Anon had a mental breakdown perhaps due to 'black' corporation Japan is famous for: Demanding, lousy pay and overbearing boss and assholish colleagues. He actually wanted to take it easy but cannot and had his mentality reduced to a kid and wanting a mother to pet him, thus going after the motherly Reimu.
I feel bad for the anon. Him playing the part of an angry yu did make an interesting twist on the slaughter. Bonus points for the sudden auto-death, too.