Why did Dave?

He needed practice before the final showdown scene.

because, fcuck it

Judging from how the townspeople in the bar also stared her down, it seemed like she was new. Maybe to get rid of new people?

The commentary mentions that in the original script, Dave is one of the old gods/dark forces and he is around keeping the town under mind control to cooperate and keep the sacrifices going.

Since they sort of wrote that out, I guess Dave doesn't like new people being around because they could blab and they aren't under the same mind control as the rest of the town.

The idea I got was they released the old gods/dark forces and they town burned Dagmar's family as the first offering to appease them. Then Dagmar became a tool of the sacrifice the subsequent 2 times in 1919 and 1949. But they didn't want to kill the new family this time.

The psychic couple kept telling the family it is NOT Bobby. It wants you to think it is. So I don't think it is Bobby at the end, just a trick.