Why does Ross have doubts about accepting the grooms as murderers?

Here it is: At the end of Act 2, in Scene 4, there is this dialogue:

ROSS:

Is't known who did this more than bloody deed?

MACDUFF:

Those that Macbeth hath slain.

ROSS:

Alas, the day!

What good could they pretend?

MACDUFF:

They were suborn'd:

Malcolm and Donalbain, the King's two sons,

Are stol'n away and fled, which puts upon them

Suspicion of the deed.

ROSS:

’Gainst nature still!

Thriftless ambition, that wilt ravin up

Thine own life's means! Then ’tis most like

The sovereignty will fall upon Macbeth.

Ross has his doubts because it doesn't make any sense. A murderer has to have a motive, and as he asks, "What good could they pretend?" What in the world could they gain by doing such a thing? (Weirder still, although he doesn't ask it, why would they kill the King and then go back to sleep!?). The answer, from Macduff, is that they were secretly ordered to do the killing by the King's sons Malcolm and Donaldbain. That explanation, too, makes no sense to Ross, for their father was loved by them and was all they had.

Leave it to the Macbeths to come up with a plan that nobody in his right mind would believe.