Why is everybody talking about Under The Red Hood as if it's some major Batman magnum opus that's due for a live action version? There are dozens of stories that are more popular and critically acclaimed to be adapted first.
Also, arguing whether or not Batman has killed in films before is pointless. His no-killing code has been an integral aspect of his character in the comics for decades now. If they ditch that in the new movies, they are going against an aspect of Batman's character that is held to be significant and essential for many fans. This is not a superficial change, like changing his logo or skin colour. These fans are justified in being upset and rejecting this change. Simple as that.