吧 with the passive voice I'm curious if it's grammatically correct to make suggestions with passive verbs. Like "Let's get killed".

All I can think of is a suggestion with the causative verb "让我死吧".
Yes, it would be grammatically correct but wouldn't sound right. 

However,

Using (ba) which is called a tone softener is used to add causality or make the language sound less severe; adding it on to a sentence such as 'Let's get killed' would sound funny, almost jokingly.

For example:

你看书吧。meaning: Read. To make this seem more polite, consider adding 请.

你看书。（！）Read(./!) normal severity

你现在看书！Read now! extreme severity