Well you can look at it two ways.
Such insults are low effort garbage. The people who make them are less than that. Don't concern yourself with their idiocy.
Their life probably sucks if they are saying crap like that to you. I mean, who grows up with so little guidance that they think it is okay to wish cancer on somebody? If it pleases you, just think of that and let the thought that their life sucks and they are getting what they deserve for saying crap like that.
or 2.
You can take the more empathetic approach, they probably don't mean what they said. They are just super angry and their life sucks so they are expressing all of that by saying the worst thing they can think of.
They don't actually care what it means. They are only saying it because they want to say something that is as bad as they are angry. It changes things when you think this, because then it isn't about the insult at all, but about them. And it becomes easier to ignore knowing they don't actually want or believe what they are saying.