The reason the Marines should've wanted to kill Ace isn't because he had the will of D, or that he was Rogers son. Or even that he was one of Whitebeard's favorites.
They should fear him because his mother, Rogue, is the strongest character in the entire world. And frankly, I'd wager she's amongst the most powerful in all of fiction.

"Hillan, what the fuck are you smoking?"

Simple math.

Rogue's one proven feat was holding Ace from getting born by an extra 11 months, making her total pregnancy 20 months. That doesn't sound so impressive, right?

Wrong.

No woman in modern times has gone more than 14 days 'overdue' with the birth of her child, essentially nobody that does survives. But, for the sake of simplifying the math, let's say one in ten women would survive holding the baby in for one extra month. And then we multiple that number by ten for each month.

The end result of that, over 11 extra months is 100,000,000,000. In other words what it took to do Rogue in, was enough physical strain to have killed Hundred Billion regular women.

Let's put that into One piece terms. Whiebeard, let's say the injuries he sustained at Marineford would've been enough to kill 10,000 men. That's impressive, sure! Let's say that over the course of his long life, he got three-times as injured over time, that's 30,000. That's incredible.

But it's still less than a Thousand of a percent of the physical strain Rogue had to take.