Hey Hive account@anonym0us, good job. I understood the concept and the purpose of this bot.

A few steps that can improve the security:

1. You can create a community account.
2. Have your users authorize your account to vote of their behalf upon registration using SteemConnect. A posting authorization link can be like this https://steemconnect.com/authorize/@YOUR_COMMUNITY_ACCOUNT.
3. Check for who have given you access and only register them.
4. Now you can vote on their behalf using your own posting key without saving everyone's private keys in database.

Keep up the good work!