Can an act be objectively and intrinsically evil, and the doing of it be not sinful? Yes. How? If the person does not know that the act is evil. Say that a child takes the Lord's name in vain, having learned the practice from his parents, and never having been told that doing so is a violation of the Second Commandment. What the child is doing is intrinsically and objectively wrong, but the child is not morally culpable nor is the child sinning, for the child does not know that taking the Lord's name is wrong, nor could the child have known this by any means available to it. The child's ignorance is 'invincible ignorance'. So, not all evil acts are sinful acts.
What about the Muslim suicide bombers? God has given us each a conscience. The reason we can be held responsible by God for what do is precisely because we have all been given a conscience. It is possible to sear one's conscience, but one is morally culpable for doing so, because we know deep inside that we should not sear our conscience. So, if the Muslim suicide bombers have seared their consciences, that doesn't get them off the divine hook, anymore than it gets a drunk driver off the hook; the drunk driver is responsible for what he does in his state of drunkeness, precisely because he knowingly and culpably put himself in his drunken state without ensuring that he would not get behind the wheel of a car in that state. Likewise, the Muslim suicide bombers with seared consciences are for the same reason still morally responsible for all that they do in that seared state, even if at that time they no longer know that it is wrong. But if the Muslim suicide bombers have not seered their consciences, then they are sinning when they pull the detonating cord in a crowded restaurant or marketplace, or fly a hijacked plane into a building, for in that case they still know that the murder of innocents is wrong.
-A8
If I am 18 months old or I have an I.Q. of 50 or I'm hallucinating and I think you're an axe murderer, and I shoot you dead, killing is still intrinsically evil and murder is still a sin, but I am not fully morally guilty because I haven't sufficient intellect or will to be held entirely responsible.
That's why we can say that some acts are always intrinsically wrong, but only God can be the ultimate judge of individual guilt.
As for the jihadi suicide bomber: all I can say is that when God, who sees the heart and knows all things, and can neither deceive nor be deceived, makes His judgment at the end of time, all of us, whether saved or damned, will acknowledge that His judgment is absolutely perfect.