When having a debate, why do some people turn to insults?

It is called argumentum ad hominem.  When you cannot deal with the facts, you attack the opponent personally.

Insults and abuse only come from Losers

Is called ad hominem. Its Trump’s entire political philosophy.  

Because they dont have any argument

They don't have facts or logic.

Because it is difficult to debate if you're on the defensive. It is a cheap tactic used by people who are not confident in their beliefs.

It might sound like an insult to you, but be a fact you deny.  They might have been listening to Trump because I think he insulted every woman in congress at least once.  

Human nature. It's not always good... Also, they were probably insulted themselves. Nastiness, as well as niceness, has a way of spreading itself around.

David is spot on correct.

Because they're douchebags.