Utilitarian Respect For Persons

A quick thought: Some object to utilitarianism on the Kantian grounds that it fails to "treat each person as an end in themselves, and never as a means only". For example, utilitarianism may instruct us to sacrifice some people as a means to benefit (more) others. But I'm not sure this objection really holds. Utilitarianism takes the interests of everybody into account, and thus never treats anyone only as a means. Even the sacrificed people were considered as "ends in themselves", their interests included in the utilitarian calculus. It just turned out that their needs were outweighed by other people's. So they were treated as a means, granted, but not as a means only. Now, it's not so clear that this weaker result (i.e. treating people as a means in addition to an end in themselves) is something worth objecting to. People benefit from other people all the time. Ask a stranger for directions and you are using them as a means to your own ends. This seems morally innocuous. But perhaps that's because the other person (tacitly) consents to being 'used' in such a way. It seems more problematic when people are used against their will. But then, utilitarians would agree that it's unfortunate when anyone's interests (or autonomy) are sacrificed. It's a bad thing in itself -- but this harm might be outweighed by a greater good. The real question seems to be whether it's an absolute, overriding evil to use someone as a means against their will. But the answer to this is obviously "no". If someone knew the code to deactivate a nuclear bomb which threatened to kill millions, but stubbornly refused to share the information, one would be well justified in tricking, drugging, or otherwise forcing him to share the code against his will. So, what's the objection, exactly?