I don't think it was stupid.

Yep! I never wondered it. But yeah! Voldemort should've known that Snape isn't faithful. Good one!!

Ok maybe stupid was the wrong word for it but it just didn't seem fitting to their characters the way they died.

well, i think i've got sumthing to say.
If u re-read the sixth book (Half-Blood Prince), u'll get the answer to ur first query.When Bella asks Snape about dat, he replies dat Voldemort did nt dare reveal him to Snape thinking that Snape's turned to Dumbledore's side.So Snape didn't know that Quirrell was acting on Voldemort's orders and just thought it was the "greedy and unworthy Quirrell attempting to steal the stone".
Snape had told Voldemort the same thing, and it seems Voldemort believed him.

Then the second one.Death doesn't always come according to what a person was like when he was alive.Sumtimes, it happens all of a sudden, sumtimes, very slowly.So, there is nothing weird about Fred's death.& Sirius was hit by the killing curse which caused his death.He just fell into the veil after dieing.
This is my opinion, so plz don't feel offended

@Hermione30 No worries, I'm not offended. I'm not quite there on my books yet. Just started reading the 3rd one. It seems to take me ages to read them now lol. Yeah I guess its just me. It really irritated me the way she killed Sirius and Fred but that was just me. But no offence taken it was just to see what other people thought :-)

@Laura_T thx and ya, JKR has taken pretty different methods to kill them.But, thats she's famous for - her creativity!

This is very, very true. She is good. I wonder what she'll do next? Hmm

hmm..ya, HP is really gr8!And i was disappointed with its end.I mean, who'd say no to another HP book?

This is true. I don't think anyone would. But would she do a grown up one or a prequel to when his parents were still alive? This is the question lol

Well their deaths are a little strange, like ppl are gonna aske How did Sirius die? And imma be like he fell through a veil, they gonna be like WHAAA? though yeah Fred was killed by a colapsing wall, but it was cause of an explosion, not THAT weird