The author does not actually explain what he means by this line.  However, it seems to me that it means that they both hold grudges very firmly -- that they do not find it hard to hate others.

We do not really see this with Hauchecorn so much, but we clearly see it with Malandain.  They only quarrel that the two men have, as far as we know, is over a business deal from some time in the past.  Because of this seemingly unimportant thing, Malandain hates Hauchecorn enough to falsely accuse him of a crime.  He then keeps the pressure up on Hauchecorn until he cracks.

That's pretty hateful, and it shows why Malandain, at least, is a good hater.