The person you’re replying to is equivocating between the violence of slavery and the violence of a slave revolt, ie the Haitian revolution.

Dessalines was a former slave who became a leader of the revolution. He order the massacres of French colonists out of fear that they, with support from the French government, would overturn the revolution and reimpose slavery. His brutality was a reflection of the incomparably hellish conditions and violence that Haitian slaves endured.

That said, the questionable morality or necessity of his actions are not the reasons why he’s held up today. People don’t see him up as a figure to emulate. Instead, he’s a symbol and reminder to the perpetrators of imperial violence of the consequences that they will inevitably face.