If you read the translated light novel, you can see it’s not that complicated. Spoiler...
In the anime Diablo is only lamenting that Beretta (who is of his demon line, since he is the originator for black) was able to be summoned and serve the great Rimuru 1st.
Diablo is surprisingly cute and often childishly competitive in his jealousy regarding serving his self declared master. The back and forth with Shion on this in the web and light novels is also too funny.
I just googled Beretta and I'm glad that I was right haha so he is the one inhabiting the golem.
I read a bit of the novel (not everything because I want to still look forward to the manga and anime) and I can say that Diablo is one of my favs
In the anime, in episode 23, Diablo said "What a horrible blunder. You finally summoned me and my own family got ahead of me" What does that even mean ? Didn't Rimuru just help the kids? When did he summon Diablo? Is it about summoning a demon to inhabit the golem ?
But Diablo is a much higher existence that required so many sacrifices so i doubt that much could've summoned him so why is he lamenting the fact that he didn't get summoned ? And even if he did get summoned he would've been in a weak body seeing as there is no appropriate sacrifice and also he would've been bound to the golem and to serve the tiny fairy demon lord and he wouldn't have been able to serve Rimuru which seems to be his goal from the beginning. Did he count on being summoned and then breaking from the contract to serve the demon lord and get to Rimuru ?