What is the difference between the future and present tense in the following sentences? Which tense is more preferred?
Quiet or you die!
Quiet or you'll die!
Hold still or I kill you!
Hold still or I'll kill you!
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As has often been pointed out on ELL, English only really has two tenses (past, and "not-past"), so there's really no "grammatical rule" in play here. It's more a matter of idiomatic preferences.
One alternative phrasing I didn't give in the above link is "If you cancel, I go instead". Without more context, that version might look slightly odd, but in some cases present tense is actually preferred for the "consequential/subsequent action". Consider, for example,...
It's not easy to identify general principles about when native speakers tend not to use the modal will to indicate "future tense" - but it's certainly true that overall we do use it more often that not.
I think will is less likely to be used in contexts where the speaker wants to emphasise the equivalence of two actions (since the first one is invariably already expressed in present tense). Thus, for example,...
It's worth noting that will also suggests volition and/or inevitability. To some extent, my examples can be seen as "statements". But OP's examples are more likely to be uttered as "threats", in which context will is much more likely to be included. In the final analysis though, it's normally just a stylistic choice.
Since the killing would take place in the future, I think it's best to include the will.
Also, if I was interested in strict and proper grammar, I'd probably change the first one to:
Be quiet or you'll die!
That said, kidnappers, carjackers, bank robbers, and hostage takers who make these remarks are normally rather jacked up on adrenaline when they make such threats, and therefore don't tend to worry too much about correct verb tense, so you might find either of your alternatives in a book or movie script.