Alucard cried in Castlevania due to the overwhelming grief and profound sense of loneliness following the death of his father, Dracula, which signified the loss of the last remaining half of his family.
The Devastating Loss of His Father
Alucard's tears were a powerful expression of the deep emotional anguish he experienced after the climactic battle. Although he and Dracula had been at odds, they briefly shared a tender, loving moment before Dracula ultimately crumbled into dust. This fleeting connection made his father's demise all the more impactful, as it meant the death of the only remaining direct family member Alucard had left, following his mother Lisa's earlier tragic death.
The Weight of Solitude
After the intense events subsided and his companions, Trevor Belmont and Sypha Belnades, departed to continue their journey and lives, Alucard was left in profound solitude. He retreated into the vast, empty halls of his ancestral home, Dracula's Castle. In this quiet, desolate environment, the full weight of his monumental losses, coupled with his immortal existence and the prospect of an eternity alone, became unbearable. This led him to break down into tears, a raw display of his emotional pain that resonated deeply with audiences.