Tom and Huck decide to keep mum about the graveyard incident because they are afraid of the wrath of Injun Joe. If the boys do tell and things happen as they should, Injun Joe will hang, but if, for some reason, that didn't happen, they are afraid of reprisal. Tom and Huck figure that since Muff Potter was drunk and had "just got (a) whack when Injun Joe done it", they are actually the only witnesses to the crime. If they tell, Huck observes that Injun Joe would "kill us some time or other, just as dead sure as we're a-laying here...we got to keep mum...that Injun devil wouldn't make any more of drownding us than a couple of cats, if we was to squeak 'bout this and they didn't hang him".
Tom and Huck make a pact to keep what they have seen a secret. They feel that their oath of silence is important enough that it must be sealed with their blood. Tom writes their declaration on a piece of pine shingle, and the boys prick their thumbs with a sewing needle and squeeze out enough blood to seal it with their initials (Chapter X).
They were both afraid of Injun Joe killing them. If he was not killed, they knew he would come after them