Bio Hunter, Kyoushoku Soukou Guyver (2005) Recommendations
Both titles feature a protagonist that can transform into a super powerful humanoid being and kills various people with ultra-violence. The Demons in Bio Hunter have monster designs similar to the Zoanoids of The Guyver franchise. Both are action-packed anime with violence, blood, and gore. Difference between the two is that The Guyver is more sci-fi while Bio Hunter is more horror. If you liked one, you'd like the other.