What is the work?
Kamen Rider Kuuga, the first installment in the Heisei era of Kamen Rider. The basic premise is that an ancient battle-hungry race called the Gurongi are unsealed after the cave they were imprisoned in is excavated and begin killing humans for their "Gegeru" game. To combat this threat, Yusuke Godai inherits the power of Kuuga to fight them with assistance from the Japanese police.
Who is the villain?
N-Daguva-Zeba, the leader of the Gurongi and main antagonist of the show. As one might expect from the ruler of what is essentially a tribe of serial killers, Daguva's a pretty bloodthirsty guy and is the primary cause of almost all of the misery in the show, wanting nothing less than to corrupt humanity into a race of monsters like the Gurongi.
What does he do?
Daguva is the one responsible for instigating the Gurongi's Gegeru game, in which the Gurongi mass murder humans for sport. While Daguva had little direct involvement with the individual Gurongi's killing sprees, he's the one who enforced the Gegeru and made them participate, making the thousands of deaths caused by them ultimately attributable to him. In addition to his callousness over human lives, Daguva cares little for the lives of the people of his own tribe either, personally murdering over one hundred Gurongi for being too weak to participate in their killing game.
Despite not being seem for most of the series, after much buildup, Daguva appears to confront Kuuga once he's killed most of the Gurongi. After delivering a brutal beating to Kuuga, Daguva burns to death countless innocent people around them to spite him before leaving to cause more havoc, his end goal being to corrupt humans by bringing about the "Ultimate Darkness" so they'll become monstrous, bloodthirsty creatures like the Gurongi.
Since he sits at the top of the Gurongi's hierarchy and is the one who unleashed the Gegeru, almost all of the deaths and actions of the Gurongi are attributable ultimately to him. This includes the massacres conducted by the other two PE Gurongi, Go-Jaraji-Da and Go-Jaaza-Gi.
None. He's nothing more than a bloodthirsty sadist who enjoys murdering those he sees as "weak", not placing any value on the lives of humans or his own tribesman.
Big yes. He has one of the highest body counts in the entire franchise.