You know, I'm not so sure it was bad writing. Lulu gets this power, that at the time to him seems invincible and amazing and blah blah I'm totally going to kick Britania's ass in one week, watch me. He's so drunk on suddenly having this power, when he had absolutely nothing to fight with, that he loses himself. The chance for revenge on Britania was right in front of him, and instead of thinking things through he just...went with it. I mean, just look at how he reacts later, when just thinking of killing Clovis makes him sick. I'm sure that he regrets the incident now, and probably wishes he had approached the situation in a manner that allowed him to keep his identity secret, allow Clovis to live, and obtain the information, but what can he do? It was a blast of arrogance that is still costing him now.
I guess what I'm trying to say is that it does make sense to his character, being the difference between his carefully thought out plans and his spur-of-the-moment decisions. It was the same when he 'killed' the emperor before he could obtain information; he didn't think it through.