I can't speak for "gateway behaviour" but it seems that there is a correlation between animal cruelty and serial killers. Then again, there is a stronger correlation with childhood abuse, and obviously many abused children grow up to be far more humane than their childhood abusers.
Stepping on ants is, at least, not torture, and way better than hassling mammals. For the ant, darkness would loom and then becomes total. I personally would rather die quickly under a giant's foot than, say, suffering severe degenerative diseases in an old person's home. Many don't think that insects have qualia, believing that they don't experience their lives. I think they would experience their lives but I have killed countless of them myself when they have invaded my kitchen, so I'm no saint.
Invasive, fast-breeding animals have a special ability to erode the empathy of even the most docile of humans!