User blog comment:B214/Shin DxD 1 discussion/@comment-2607:FB90:3851:A5D3:0:14:ECCB:7501-20181029115052/@comment-36663391-20181029115957

The fact is Issei want becoming a indipendent King and Rias is his Master/King. The other reason is Rias is becoming indipendent by Issei and according to Indra she is be stronger than before because she can't have the Issei's help. Issei need to defeat her for this reasons.