You were Ainz's enemy as u were one of the humans who attacked Nazarick. But u hated humans since they were the ones who ended your parents life, so u disobeyed orders by not killing any of the servants only injuring them a little before leaving them be.

After a while the humans lost and u were captured. Ainz wanted to kill u but he found out about how u didn't kill his servants and decided to give u a choice

"Human...I will give u chance do u want to die as a human or join me as a demon?"