Some animals have horns, like built in spears that give them an upper hand. Naturally, they become good fighters, regardless of the why.
Sometimes rams aren't as obvious as that; sometimes the horn is a choice, a decision made from practicality and for brutality.
Shuten isn't much of a martial artist, isn't full of pretty kicks and spins. He's too direct for acrobatics. A spear on the other hand suits him just nicely. It's not too weighted and heavy, and like a ram's horn, it's good at piercing through things.
If Guen could be considered a boar, charging into battle, with only himself as a weapon, then Shuten is the ram, bringing a horn to the battlefield.
Shuten likes difficult battles, a good challenge, but he also likes brute force that makes it easy to tell who won. The loser is left behind, and the winner is definitely not a corpse.
Shuten has always been a warrior; bloodshed has never bothered him. His own strength has been his primary deciding factor.
He's with Hiryuu out of some sort of loyalty thing, but mainly because this path gave him more strength, a battlefield, and was definitely a path he chose for himself. The pros far outweighed the cons in Shuten's book. Absolute loyalty was a bit of a stretch to gain from Shuten after all.
And in some ways, he kind of liked it here after all.