@Nk9bjP4A
If I remember correctly, if a master want to make a contract with servant, his 'presence' must be apllied to servant by any ways.
Since slime had 'sex' with master, she had this contract in her body and when she exploded on that spider, her contract was over all on spider's body, thus making her become contracted by indirect way.
@Nk9bjP4A Been years but IIRC she was drawn to him from the start, but because of her history (She's a rare, old, monster that already had some thoughts of her own, which is something that the generic mobs lack)
she 'thought' that she wanted to dominate and monopolize him.