Possibly a password theft if your password is too easy to guess (several words you always use) but in order to steal your coins, the hacker must also have access to your private key. In fact there is no 100% sure way to make sure that no one else have your private key unless it is generated by you manually and kept offline secretly

The entropy in core client should be strong enough to avoid key collision, but the key should be generated offline and kept offline most of the time