Al was a human who'd died and went to Heaven, so he's a not part of the Heaven's hierarchy.

His background is actually more or less the same. He did murder a lot of people when he was alive, but the only reason he's in Heaven is because it was only BAD people he killed. Legitimately bad people. People who definitely wouldn't have made it into Heaven.

He was also a social activist and used his radio show to both entertain and keep his audience informed of current issues. He was on the front lines of a lot of local, political issues, challenged the status quo, and helped his community in a lot of ways.

He murdered a lot of people,

buuuuut

he helped a lot more people.

So when he died, up to Heaven he went. (He was very surprised too.)

I kind of see this loop hole in the same way that Adam gets to get away with killing mercilessly, via the Exterminations, and isn't condemned for doing it. To Heaven, the people he's killing are Sinners--you know, the bad people, the ones who "threaten" Heaven's power--thus it's for a "good cause" and can be "excused."

The people Alastor murdered were bad people, and like Adam, he enjoyed killing them. He enjoyed it immensely. It was his own, dark way of performing social justice, his own way of fighting for a "good cause," and he doesn't regret any of it.

So, technically, when he died, he fell under the same little loop hole Heaven made for Adam, and despite knowing that Alastor killed people when he was alive, they can't question his right to be there.

Afterall, Heaven doesn't make mistakes.