Why do Christians think atheists are out doing evil things in their free time?

Today I was playing my 3DS and listening to a Sam Harris podcast in my living room. He was interviewing Rick Caruso and they're talking about the LA fires.

My mom walked through to make some coffee and she over heard the podcast. She ended up sitting down and getting completely engrossed in the podcast. At one point she even hunched forward to really pay attention like a gamer doing a hard boss.

My mom said 'I remember this guy, he used to be a famous journalist.'

I had to correct her and say 'no, thats Sam Harris a famous atheist.'

She was incredulous 'hes an atheist and hes talking about charity?'

She couldn't fathom that someone who doesn't believe in God can be charitable. It was eye opening because it feels like to me she thinks atheists go around punching infants in their free time.

Why is this? Its wild to me.