I'm a little surprised by some of the answers here.
To the OP, if you feel it goes against your consciounce then don't join the military unless
you find peace on this matter.
To a few others, unfortunately killing sometimes cannot be avoided, for example during
world war 1 and 2, if people refused to fight Germany, then Europe would be a communist/nazi regime
and there would be no Jews left at all. I think that there are times when good is called to overcome
evil via the method of war, unfortunately that involves killing. It's also biblical look at all
the times God called His people to utterly destroy the enemy in the OT.
Then there is the issue of police killing an armed gunman. What should they do, stand back
and let these people continue firing into crowds of people. Police are law enforcers and protecters
of a country. The military are law enforcers and protectors of borders and also on an
international scale.
I think the difficulty is that as part of any protective organisation, you may be called to
fight for something that you feel is wrong. I don't know the answer to that one whether
at that point you might have to reconsider your military career. I don't think joining the
military should automatically mean you leave all decisions up to the commanders and
obey without question. So for example if soldiers are told to bomb a village of civilians,
questions need to be asked. There are many who have found themselves convicted
of war crimes because they didn't question and went along with the crowd.
Joining the military does not negate responsibility and accountability.
I think for a Christian they need to take this to God and find if it is right for them and also
keep God close during their military career. It could in fact be argued that the world would
be a better and more just place if more born again Christians were in the military, especially
if they find themselves in the high up decision making roles.
ps I don't know about other country's, but the military in the U.K. are also used in peace
keeping roles.