Before Steelwill's graphical update, he had a mohawk and a flail that shot magic projectiles at enemies, and had a unique death animation along with his fellow bodyguards. After the update, he now has a hammer. Along with the graphical update, his death animation is similar to that of a goblin.
Although the examine text says "A battle-honed goblin", Sergeant Steelwill, along with his fellow bodyguards, have considerably much less health then the goblins fighting Zamorak's forces outside.
Concept art for the upgraded Sergeant Steelwill, alongside his fellow sergeants.