[fulafel]

Do you mean the worst that could happen to you personally or the worst for everyone?

When your device is compromised by hostile actors I guess it depends on what your nightmares are, but getting framed for child pornography and/or blackmailed for it is a popular one. Or getting your cloud accounts hijacked and all your stuff compromised. Or getting the bad guys access to your employer's network. Etc.

Collectively a widespread Android device botnet could take down a lot of infrastructure, or start a war, or ruin everyone's days with ransomware. I'm sure more imaginative people have thought about it.