I really think Harald influenced Shamrock and changed his perspective a lot, in the same way Rocks changed Loki. There would be no reason for Sommers to assume Shamrock might have visited Harald's grave if they weren't friends of sorts. It just parallels Loki and Shanks nicely.

Harald became a father figure to Shamrock and his morals has rubbed off on him. Shamrock won't be pure evil.

Some predictions on Shamrock

People have noted he doesn't seem to be all that evil, at least compared to someone like Garling. I think we all know Garling probably is an awful piece of shit father who probably beats his kids or some shit, but after Harald made whatever deal he made with the WG, Shamrock was assigned to work with him. Harald and Shamrock became friends, and eventually developed something akin to a father/son relationship, or at least Harald mentored Shamrock a bit, and some of Harald's ideals about peace and cooperation eventually rubbed off on Shamrock, whether he wanted to or not. Harald unintentionally became the father that Shamrock never had. This also makes the eventual fight between Loki and Shamrock have a deeper layer of meaning. Harald's biological son vs his work son who he probably spent more time with than Loki due to his duties.

The other Holy Knights clearly consider Shamrock to have been close with Harald, as Sommers assumed he might have visited his grave.