He might have been in battle before. But the early events of TFA may have been the first time he’s been ordered to massacre innocents. For all we know he’s killed plenty of Resistance soldiers before.
But either way we can’t really make assumptions about these things before we have all the facts in.
He was having trouble in the battle before he was ordered to kill any innocents. Another Stormtrooper died in his arms and he reacted like it had never happened before. There were other references to his being fairly new, if I recall, which makes sense given his background in sanitation.