The Seamus Heaney translation says Grendel "nursed a hard grudge." When I see that, I understand a couple of things about Grendel's heart. First, he's bitter, and bitterness is something which hardens the heart. Second, he's nursing whatever unspecified grudge he's got. Nursing implies he's figuratively feeding it, keeping it healthy and growing and alive. Finally, it's a hard grudge. This is not something which will go away on its own over time--especially since he's nursing it.
Given that, Grendel can laugh all he wants and it will never be a joyful laugh. He is likely laughing at the fear and despair and grief of those whom he's terrorizing. This seems similar to so many of the classic movie villains who laugh (or chortle or snicker or roar) when they have their prey in their grasp. It's an evil, joyless laugh, and it must be the same for Grendel.
The epic poem Beowulf draws a sharp contrast between the human characters and the monsters they do battle with. Grendel, as a monster "spawned in that slime," is fundamentally different than men--he is cursed by God and unable to understand what the lives of human beings are about.
We see this early in the story when Grendel is enraged by the men's behavior in Herot. In their mead hall, the men celebrate together, experiencing a kind of fellowship that Grendel could never know. They also listen to the religious stories of the scops, stories that don't hold a place for a creature like Grendel.
So when the poet writes about Grendel's "laughter," he is juxtaposing Grendel's perspective with that of the men in the hall. The men experience happiness because of their togetherness and their sense of spiritual belonging. Grendel, "forever joyless," can only experience pleasure at which his "heart laughed" when he engaged in sinful, violent behavior. That's why we hear about his heart laughing moments before he kills one of the Geats.
He won't laugh long, however, because his next intended victim is Beowulf.
Not all laughter is the product of joy in its positive sense. Laughter can also rise from malicious delight. Such delight is the ape of true joy, it's caraciture.
Grendal is a stranger to true joy, to true happiness, which the Beowulf poet would ascribe to Christian virtue. Grendal's delight is in cruelty and thus his laughter is cruel and heartless. It is a mirthless reveling in the grief and destruction he brings.