"Beware of Grab".
Why "beware of Grab" if neither of them did anything that affected you as a passenger, what was so "scary" about them?
How could you "know" that one of them wasn't a Thai and what is the relevance of the other "looking Chinese Thai" as a huge number of Thais actually are?
Why did both of these drivers in particular need background checking and why on earth did you order Grab's if you were so concerned about the alleged lack of background checks that you knew about when booking them?
What was Grab's reaction when you reported their "menacingly scary" facial appearances and the action of one not leaving immediately after dropping you off?
Were these two drivers the only Grab rides you have had and therefore you experienced a 100% "bad" hit rate or have you had other Grab rides that went unremarked that far outweighed your irrational dislike of the appearances of these two?