どう can, colloquially, be used to express one's opinion of a thing being "questionable", or "dubious", without outwardly nor directly criticizing it either.
It takes this meaning when used more "affirmatively" in the sentence, a bit like interrogative words may take a critical meaning when put in rethorical questions in a language like English.
In other words, akin to how
What do you say ? asks a genuine interrogation while
What do you think you're saying ? indirectly states a reproach,
("What's this") most likely asks what something is while
どういうことなんだよ！ can express critical bewilderment ("What the **** is this !?").
Similarly, people will often utter
どうかな to (inwardly) express doubt (疑問【ぎもん】) about the validity / reasonability / etc of something.
どうかと思う expresses the same doubt on the validity / ethics / etc of the subject.
The expression "not sure what to think about ----" (with, usually, a rather doubtful and/or critical tone) I think conveys the idea pretty well.
So in your example,
すぐ死ねって言うこと is what's being questioned by the speaker. You could replace the sentence for example by
すぐ死ねって言うのどうなんだよ and it'd become a harsh (yet indirect) reproach of "instantly saying 'die'".
すぐ死ねって言うのどうかと思う or even more explicitly
すぐ死ねって言うのどうかと個人的に思うけど / etc instead conveys the speaker's mixed feelings about "instantly saying 'die'" in a much more indirect thus soft and gentle way.
"Let me let you know I have rather mixed feeling about", basically.
Eventually the use
~☆ hints that he|she is making his|her criticism as lightweight and indecisive as possible. I'd wager he|she doesn't mind it much eventually, is just lightheartedly bantering, or is quite afraid of and cautious not to ire the person who said "死ね。" for example.
I'm not sure what to think about instantly saying 'die' ;)
What's up with that immediately saying 'die' :/
for example would sound like legitimate translations to me, especially tone-wise.
You can find in the 大辞泉 "having doubts, not being sure" and "finding it odd, distancing oneself with" as 2 separate definitions for
Interestingly, you find a mix of these two (the amount of which is left to context interpretation) in typical uses of
I personally think the root feeling is basically the same though, and as such didn't distinguish them in my above explanation.