Did he get hard at the thought of killing someone?
I... maybe?
Since you posed the question, I'm reminded of a segment in
Sensei's White Lie, where the heroine remarks that she hasn't had a period in the longest while (because, presumably, the life of sexual trauma and the consequent stress that she's had since shortly before the beginning of the narrative). She was also unwittingly building a rapport with one of her homeroom students up to that point, and upon considering that she desired his company (as in, at the point they found themselves kissing for the first time), she found herself menstruating again (that, and/or she experienced a genuine arousal for the first time in a long time, this being signified with the invocation of the idea of menstruation).
It's odd how I've seen this kind of sexual symbolism-- a linkage between reproductive function and will to live/something of that sort-- potentially twice in manga. Certainly, I personally haven't seen this kind of literary device in American stories, and I reckon it's less common.