To begin with, recall that Kantian deontology is a form of deontology that places absolute moral value in the agent's intent. One problem with this emphasis on intention is that it rules out the moral relevance of external circumstances. In particular, it doesn’t seem to make sense of the fact that we judge people even when they aren’t fully responsible for the outcome of their actions.
People may think that the successful murderer is worse than the unsuccessful one, even though the difference between them was due to luck. For this reason, this is often called moral luck. Kant thinks morality can’t depend on such things as luck because he argues that moral worth is only in the intentions.
There are many other instances where moral luck plays a part.
Imagine you decide to dedicate your life to science. You figure that you need to cut off all contact with your family and friends in order to put all your energies into making discoveries.
If you manage to advance science through your efforts, then it may have been worth it. But perhaps you’re just not cut out for it, and you never make a lasting contribution to knowledge.
Most people would think highly of you if you were successful. But if you were unlucky enough to fail at your life’s work, then you might be judged badly as someone who abandoned their family for no good reason. As you can see, moral luck plays a part here.
In the above example, whether or not you have natural talents (e.g. for scientific study) is out of your control. There are many other factors that could be out of your control.
The circumstances around your birth play a role in the opportunities you have to act morally.
Sometimes it seems like you should do something even though it goes against certain (Kantian) duties. For instance, it might be the case that saving human life requires lying or stealing.
A Kantian deontologist seems to be unable to resolve the conflict between the duty to save life and the duty to not steal or lie.
Another problem for the Kantian deontologist is that they seem to be committed to the idea that ignorance is good. That’s because you can do bad things and still be a good person as long as you are unaware of the moral impact of your actions.
Since intentions are the only thing that matters for the Kantian deontologist, it’s better to not know about such global political issues. That’s because you can still have a good will if you unintentionally contribute to international abuses. There are many other examples like this.
Imagine that you manufacture snacks. You produce food that has a very high proportion of salt, sugar, and fat. You do this because it’s cheaper and easier to make low quality food taste good.
If you didn’t know that excess salt, sugar, and fat contribute to serious illness, then you wouldn’t be to blame for the harm this causes.
But this doesn't mean you can be intentionally ignorant. If you tried to cover your back by purposely avoiding looking into the moral impact of your actions, the Kantian could fault you for this. For instance, if a lazy doctor intentionally failed to check up on her patients, she would be to blame for any harm that comes to them, even though she’s unaware of their condition.
Finally, Kantian deontology is often charged with neglecting the role of feelings in ethics. This criticism points out that, since duties are determined by pure reason alone, the Kantian can’t make sense of the fact that our emotional response to situations seems to be important for ethical action.
It should be noted that Kant has an answer to this objection. He says that it’s fine for our feelings to be involved, as long as it’s reason that dictates what’s the right thing to do.