I don't think it is pathetic and dumb. I used to be an ant killer and then I went to this talk on virtue ethics...

Virtue ethics is this theory whereby 'ethical people' are people who exhibit certain virtues (such as virtuous character traits of kindness, compassion, truthfulness, courage etc etc etc). On the virtue ethics picture of what it takes to be moral moral education should consist in fostering those virtues.

The thought is that you can teach your children to be kind and compassionate by such things as... Teaching them to treat the little creatures with wonder and respect. When you see an ant say 'oh look at that isn't that amazing' and then find out more about the wonderous nature of ants on the internet or from books etc in order to teach your children to have that respectful nature towards living creatures.

But... Some people aren't raised in such a fashion. Mother might go 'ugh and ant how disgusting' or one might see people casually squashing them... Often such a thing is unthinking...

I sympathize a great deal with your finding his doing that to be altogether to casual and... Slightly disturbing (or rather more than slightly disturbing).

Perhaps... You could talk to him about this? It might be... That he has never thought of that before... And that... You could educate him in the appropriate attitude towards living creatures ;-)