But murdering people in wars is quite ok, isn't it ? /s
No. Science can't say when life begins any more then it can say how much sand is needed to make a heap. Because it's a question of how we choose to define things, not a question of objective fact.
(And fyi... since what makes humans special is our rather absurd level of ability to think and learn things, human life begins with the ability to form long-term memories. Which is well past the end of infancy. This may seen counterintuitive, but there's a sound scientific basis for it.)
Which christians are you referring to? Plenty of christians disagree with you and will tell you it's a complicated issue with no black and white answer. There isn't even a completely agreed-upon definition of what a christian is[1].
[1] https://www.reddit.com/r/AskReddit/comments/6gzr6p/americans...