Hey, Severus only spits once in canon, when he is nearly knocked off a broom by Harry.
And he only exposed Remus after Remus nearly ate students because Remus couldn't be bothered to take his potion before sitting to watch the map (when he was expecting trouble around sunset). He could have exposed Remus much earlier (the same way he did in the end), but instead he just gave the kids information that could help them save themselves. The childish one among these two is Remus, who refuses to take his potion in front of Severus just to keep Severus on edge (which gets worse knowing their history).
And he only exposed Remus after Remus nearly ate students because Remus couldn't be bothered to take his potion before sitting to watch the map (when he was expecting trouble around sunset). He could have exposed Remus much earlier (the same way he did in the end), but instead he just gave the kids information that could help them save themselves. The childish one among these two is Remus, who refuses to take his potion in front of Severus just to keep Severus on edge (which gets worse knowing their history).