User blog:KillRoy231/Why did they kill Crowley?

I know Crowley's death was in the previous season and isn't the most recent big death, or the second, but I've heard two conflicting reasons as to why they killed him off in Season 12, so I'd like confirmation on which it is.

The first reason I heard is that Mark Sheppard got tired of Supernatural and left. For eight seasons, who could blame him?

The second reason I heard is that the writers didn't know what to do with him anymore. Again, eight seasons in a then-12-season show.

So, which is it? I'd like a clear answer.