The Prince interpreted

[Lisa]

No: HPFGUIDX 173402

Dana:
First of all Sirius intent is never revealed and can therefore not be
judged.

Lisa:

Sure it can -- there could be no other outcome other than for Snape 
to be killed or turned into a werewolf, and no one else to actually 
perform the killing or the bite except Lupin.  Therefore, Sirius' 
intent was clear in the canon facts.

But let me ask this question? How could Sirius possibly set Snape up
to be killed by a werewolf if Snape already knew what Lupin was?
My answer you can't trick someone into facing a werewolf unknowingly
if the person actually knows he is going to face a werewolf by
entering a tunnel, he had seen Lupin being escorted to by Madame
Pomprey, on a full moon night? So it actually wasn't Sirius who
played the trick but Snape himself who wanted to trick the marauders
by finding out what they where up to and trying to expose their
secret.

Lisa:

By finding out what they were up to?  But you're arguing that he 
already knew.  His goal was to expose the rest of the Marauders, not 
Lupin's "hairy little secret," as Snape had already seen that Madame 
Pomfrey was privy to said secret; therefore, Dumbledore already knew 
that, obviously.  What Dumbledore did not know was that the Marauders 
were sneaking out of their dorm, was that the Marauders were 
unregistered animaguses (animagi?), was that the Marauders were 
placing themselves in mortal peril by consorting with a werewolf 
while he was transformed.  Snape was not going there to confront the 
werewolf Lupin, but to expose the Marauders.  We have no hint that 
Snape knew he'd be getting into a situation that would inevitably end 
up with getting himself killed -- after all, if the Marauders could 
survive hanging with a werewolf, why couldn't he?

Dana:

What we know from the scene in Snape's memory is that 1) He knows
about Lupin and his disappearances on full moon nights. 2) He knows
James, Sirius and Peter sneak out at night. 3) He knows that he can
find the answer to his obsessive questions behind the willow.

Lisa:

Exactly!

Dana:

Although this final theory will never be canon I'd say Sirius wanted
to scare the living daylights out of Snape because he was aware of
Snape's interests in Lupin. Tells Snape all he has to do is prod the
knot and he can find out for himself.

Lisa:

And Sirius was going to ... what?  Save Snape before Lupin bit or 
killed him?  James didn't think so.

Dana:

Snape already knows but is actually not interested in Lupin but in
what James and Sirius are up to (as he wants to proof to Lily that
they are not as perfect as everybody think they are). So Snape goes
to the willow to find what the marauders are up to with werewolf
Lupin as there must be something more to it as you can't just play
with a werewolf on a full moon night. James finds out drags Snape out
without Snape ever learning the truth about them being animagi.

Lisa:

We agree on that, too ...

Dana:

Snape is humiliated by being saved by the boy that is interested in
his best friend and plays it for all it is worth by running to DD
with the story that they tricked him in order to kill him. Snape is
not yet a good occlumens at age 16 and thus DD detects he is lying.
Snape spills the beans and DD uses that to keep Snape from telling
anyone about Lupin's furry little problem.

Lisa:

And lets the Marauders get away with breaking school rules by 
sneaking out of their dorm; breaking Ministry Rules by being 
unregistered animaguses (animagi?); and allowing themselves to 
continue placing themselves in mortal peril every month?  Gee, their 
parents must be comforted.

Dana:

Snape does not want Lily to think James did something heroic because
he still believes that he did it to hide a secret from being exposed.

Lisa:

We really don't know why James did it, actually.

Dana:

Snape's body relaxing when Lily calls James an arrogant toerag is an
indication to me that he was not as upset about the whole ordeal as
we get to see of him in PoA.
The joke was supposed to be on Sirius but it backfired on Snape when
he failed to get the marauders into trouble because of it.

Lisa:

I don't think it was ever meant to be a joke on either behalf.  I 
think Snape was completely serious about getting the Marauders 
expelled, and I think Sirius was dead-set on harming Snape, to "teach 
him a lesson."

Dana:

And as soon as JKR comes out and openly state that Sirius indeed send
Snape to the willow in an attempt to murder him, it was never so and
the trick never materialized because Snape already knew. And James
saved Snape from his own stupidity instead.
JMHO

Lisa:

Precisely - that IS your opinion.  And I have mine.

Dana:

Last post on the subject because I can't convince someone that does
not want to be convinced anyway. Interpretation is very personal and
we will just have to respectfuly agree to disagree on the subject.

Lisa:

LOL -- you're the one who doesn't want to be convinced.  I have held 
theories on several things and kept my mind open enough to have it 
changed -- most notably, after reading HBP, I thought Harry was a 
horcrux.  However, there were too many good solid arguments to the 
contrary, and I was convinced to change my positions.  I've learned 
from that mistake to trust my instincts!