I think Maul would have had a much more impressive first fight against a drunk Hellboy(who Nuada only defeated with the blade shard), and a fresh Hellboy at the end who was pretty much on the defensive and running away because he didn't want to hurt Nuala.
Seriously. For a warrior prince with thousands of years of fighting under his belt, Hellboy put up damn good fights despite no formal training in bladed weaponry.
It's called gimping the villain. The villain who should be leagues better than the hero still for some reason gets defeated by the hero simply because, well, the villain should lose to the hero. Don't forget that Maul was beaten by a padawan.
Maul was beaten because he was so certain Obi had died, he didn't bother to check. He got taken by surprise, and even though I admit it was a dumb way to get killed, it was simply because he got cocky.
Nuada died because Nuala killed herself, but it was obvious he wasn't going to just beat Hellboy on speed and skill, because while Nuada was trying to kill Hellboy, Hellboy wasn't trying to do the same to him.