Casting Avada Kedavra nonverbally wouldn’t be worth the effort.
Casting a spell nonverbally gives a bit of an advantage, but is also more difficult - in some cases, it’s not worth the extra effort to cast it nonverbally. Just because Snape can do it doesn’t mean he always would, or that it’s always the best way thing to do. It’s also unlikely to raise suspicion. The Death Eaters, and the Dark Lord himself, often cast Avada Kedavra by saying the incantation.
The flash of green light illuminated every corner of the room. Charity fell, with a resounding crash, on to the table below, which trembled and creaked.”
- Harry Potter and the Deathly Hallows, Chapter 1 (The Dark Lord Ascending)
As Snape himself says, nonverbally casting spells takes a lot of concentration and mind power - so it’s not worth it to use them when there’s no need to, just because someone is capable of doing it.
“Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some ...lack.”
- Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)
When Snape teaches his class nonverbal spell casting, it’s clear that it’s a lot harder than saying the incantation - they have difficulty casting nonverbal spells that they’d cast before and could easily cast if they were just saying them. Avada Kedavra is already a hard spell, casting it nonverbally would therefore take quite a lot of power. Dumbledore was unarmed and weakened. There was no reason that Snape would need to cast it nonverbally so the Death Eaters wouldn’t be suspicious. He also wouldn’t need the element of surprise, because he and Dumbledore planned it.