In the
video you saw that the British soldiers were ordered not to open fire. The soldiers felt threatened and intimidated by the shouting colonists.
One soldier fired in panick and the rest followed.
The painting shows unarmed colonists and soldiers who open fire as if ordered by their officer (holds up a stick like he gioves the order to open fire). It looks like an execution of innocent colonists.
When the painting spread it caused much anger with the colonists.