For those who have just come around to noticing that Trump is a narcissist, his expected announcement of a third presidential run is for his own benefit nobody else's. He doesn't care if it hurts Herschel Walker's chances or hurts Republicans generally. Trump's feelings were undoubtedly hurt when members of his own party called him a loser and a drag on the party. He's angry and looking for payback. Announcing another presidential bid enriches him at the expense of others.
Running for president is surefire way to raise more money from the gullible rubes who have sent their hard-earned money to the orange-hued supposed billionaire. There are some rather vague limits to how campaign money may be spent, including after the campaign ends, so Trump will have no problem in crossing those blurred lines and spending the money for his own personal use.
The money he grifts from conservative donors is money that other Republican candidates cannot collect. Up until recently, the RNC has been paying all of Trump's legal expenses. He is a drain on the GOP finances. That suits Trump just fine.