Your right, a true Mojahid dont and should not kill innocent people. Prophet Muhammad, peace and blessings be upon him, told us that during war not to hurt the elderly, the woman, and the children. During war we should only kill people we are at war with(EG, Military).
We are judged on our intentions so maybe those who carry out the killings do not intend to kill innocent people but those innocent lives taken happen to get in the way? Allah knows best.
Hamas themselves claim to only kill those in the military and spys etc. With the Palestine issue its a heavy debate amongst Muslims some are against it some are for it.
In my opinon suicide bombings is a contradiction, sucicide is strictly forbidden in Islam. I personally dont see how killing yourself is going to get you to heaven especially if you kill innocent people.
"A soul without love cannot be elevated to the horizon of human perfection. Even if he lived hundreds of years, he could make no advances on the path of perfection. Those who are deprived of love, entangled in the nets of selfishness, are unable to love anybody else and die unaware of the love deeply implanted in the very being of existence."