The child's excuse is ignorance and inexperience.
And what do grown-ups do when they find a trail of ants marching across their kitchen counters? It's not so different.
A hurricane is unable to form a will or an intention.
The Reapers don't either. The Reapers are following a billion-year-old program.
The child may not be able to understand what it is doing and what the consequences of its actions are.
And what are the consequences, exactly? An ant hill gets destroyed. Are we really concerned about a few hundred non-sapient critters that we'll call a child "evil" for kicking over an ant hill?
The reapers have both an intention and understand their actions and their consequences, but according to you in order to be evil, they themselves need some sense of morality. That is surprising! The way most people use the word "evil", I thought that it is enough if the ones who judge have a sense of morality, not the ones who are judged.
The Reapers are an AI following a program. It does not follow that they are intelligent and understand their actions. The only thing that happens when you reach the Crucible is that they tell you that you've altered the variables; translated, you've altered their programming. It does not follow that they need to think like an organic.
The problem with a word like "evil" is exactly what you describe. Who decides what's evil? A hurricane can easily be described as evil, even with no motivation, simply because there is a path of destruction in its wake. The Reapers can be described as such too. However, ascribing some kind of arbitrary morality to a machine and determining that its actions are deliberately evil because of its programming is something I can only describe as problematic.