The Arcane Silver Sword is a rank 8 weapon and partially awakened its consciousness.
Moreover, he was connected to it and didn't want its awakened soul to get devoured by some random beast soul.
Leo used it only after making sure that it won't be a problem to use the Beast Soul Stone on the Arcane Silver.
Another thing about awakening the weapon soul was a massive temptation for Leo because only Supreme-grade weapons or artifacts could awaken the souls. If his Arcane Silver could awaken a soul, it would be on par with the supreme-grade weapons.
'At that time, I can kill the level 1 supreme-ranked beings on my own.'
Leo wanted a powerful weapon that could insta-kill the supreme-ranked beings, and with the help of the Beast Soul Stone, it was possible.
'Ding,
The host wants to use the Beast Soul Stone on the rank 8 Arcane Silver Sword. Please confirm it once again before using it.
'Yes. I want to use it.'