Great question - it was a sacrifice move on V's part if I recall correctly - he wanted to take down the current government, but not necessarily survive to see the new one (Eve would be the survivor he could never be) - I need to watch the film again to answer properly, but I believe this was the general gist of it...

This question could be rephrased as "why didn't somebody remember the exact location of their friend 11 years ago..."

@m1gp0z the MCU is the correct answer to this question, and yet nobody has provided it yet...

apparently the wall has magics in it that prevent wights from crossing, according to the answers i was given here: movies.stackexchange.com/questions/79180/…