How to make firing people suck less for them and suck more for you

I really like the idea of one month’s severance for every year’s service. Last place I worked I had to let a fellow go and that’s what I gave him, and got questioned hard about it. “But he wasn’t delivering,” was the argument against what I did. “Why would we reward him for nonperformance?” My counterargument: “We didn’t vet him well enough before we hired him. We did him a disservice by hiring him in the first place.” They backed down.