Synthetic statements are not necessary. Philosophers use the word contingent to describe something that is not necessary. It is not necessarily true that you are reading this webpage: you could be reading a printout, for example.

It is important to make the analytic/synthetic distinction in argument. If you try to argue that something is true you need to be clear about whether you are saying something about the empirical world, or whether you are clarifying the meanings of words. It would do you no good for example to hunt for a bachelor who was married to try and refute the statement. It would be no help to you to try and find a "good murder" to refute the statement that all murder is bad, because murder is by definition bad.

why should not truth be truth?
is there any alternative?

Truth for truth's sake sound good to me!