I'm not saying I share their sentiments, but some people think homeless people are ruining their country/city. They think homeless people are essentially sub-human, and by killing them, they're doing the world a favor.

So in their minds, while the homeless guys may not necessarily pose a threat of physical harm, their presence harms the country/city/world, and it's better for everyone to get rid of them.

Or, and this is probably the actual answer, these fucks are cowards who prey on defenseless people because they'd lose in a fight with anyone else.