Do you think that ADHD is an excuse for misbehaving children?

No but may be factor. Adhd can cause children to behave impulsively and cause disruptions in many settings. It is a treatable disorder for which medicatoins and good behavioral systems can greatly reduce problem behaviors. That said, studies do show that those with adhd tend to be somewhat behind their peers socially, so parents may need to help find the right structure for them.
No. . When i explain adhd to children, i tell them, " it's not your fault, but it is your problem." the more specific information you give a child about his/ her strengths & weaknesses, how medications work, & what behaviors they need to work on, the better they cope. Adhd is a neurodevelopmental disorder, not a behavior problem. See chadd.Org.
It's complicated. Oh, if life were so simple. Misbehaving children can have adhd or other conditions which influence irritability and impulse control, but that does not mean that behavior is a simple product of disease. Consequences, rewards, modeling, stress, the child's intelligence, coping skills, all play a part. Adhd children do have a higher frequency of oppositional and conduct problems, but not all.