If he new Blackbeard was that dangerous before taking the yami df, why not kill Blackbeard himself or report bb to whitebeard before bb left
I dunno but let's be honest, Shanks was stupid either way about Blackbeard. Even after he canonically knew BB was dangerous, when he went to warn Whitebeard, he did nothing about it.
Shanks didn't warn Whitebeard to prevent the war, Shanks specifically told Whitebeard that the scar he had from BB was hurting.
So if Whitebeard refuses to call back Ace or listen to Shanks, why did Shanks not go after Ace himself or send someone like Benn?
BB would have been toast.
Sooooo, honestly, I believe the true disappointing reason is plot.
If Shanks acted actually smart, the current plot would not exist. Oda often ignores little bits of logic in favor of plot.
A very recent example is Sanji and Maria. Ofc Sanji should not have fallen into the trap in the first place with Maria being HUUUUGE and Sanji having observation haki. But that is not my point. There were 2 instances in the fight where Sanji, IF he was as smart as he has occasionally shown to be, would have acted differently.
First, when he beat Marias male servants, that proves that Sanji somehow got free and battled them but didn't wanna hurt the women. So.... why did he not kick a hole in a wall and escape? Because plot.
Second, when Maria offered him a free castle wide transmission, she told him it was across the entire place, and he knew they were after Robin, why did he not warn Robin?
He could have been like, "I am trapped in this place, someone help me. Robin, stay away, they are trying to trap you specifically."
That would have been FAAAAAAAAR smarter. But the ONLY reason Sanji called Robin specifically is because Oda needed an excuse to make Robin shine.
And no, "Sanji relying on crewmates." is no good excuse, because he could have called any crewmate BUT Robin. Calling Robin into this trap also means that all the calamities and other enemies now know where she is and they know that the Yonko want her specifically. Stupid decision no matter how you twist it.
But, I'm not trying to start a Sanji discussion, those are just very recent examples that came to mind to prove that Oda often ignores logic or ESTABLISHED CHARACTER TRAITS, in favor of plot. Sanji has proven as Mr. Prince that he would be easily able to outplay Maria intellectually in that situation, but he didn't because Robin needed to shine..