I saw his initial attitude as "If someone else does my job and kills Youm for me that's great!" Then after Diablo was mentioned "Oh fuck I'm the most suspicious one if something does happen so maybe I should at least be seen trying to stop it."
If Youm dead, he fuck by Diablo too. Diablo will blame him for being incompetent noble to find the rebellion that cause ruin to Rimuru plan.
His attitude is typical noble attitude letting his subordinate do the jobs and he just waiting for the results. he believes everything will solve eventually.
But the mention of Diablo mean the standard is different. Diablo didn't tolerate incompetent and mistake, especially someone that used to insult Rimuru.
So he needs to settle it fast before Diablo notice the problem or the hell will come upon him