This is one of the most daring and generous declarations in all of scripture. Krishna says: even someone whose past conduct has been terrible — if that person turns to me with undivided devotion — consider that person a sadhu. Their resolve is right.
Why? Because the moment a person chooses ananya-bhakti, the direction of their life changes. Past actions do not vanish, but their grip loosens. The person who was walking into darkness has turned toward light. That turn itself is what matters.
Krishna does not say the past does not count. He says it no longer defines the person's future. A river that has flowed through muddy terrain does not stay muddy once it reaches the open sea. The ocean accepts it and makes it clean.