Definitions are primarily descriptive rather than prescriptive in English dictuionarfies. They are intended to indicate what meanings a word actually has in actual use, rather than trying to adjust that meaning to meet some ideas about what the word ought to mean. If a particular current meaning is offensive or controversial, or reflects prejudiced ways of thinking indicating that fact is of course part of the definition of that word.
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The exact wording of questions asked in polls, and the context in which they are asked is crucial.

It would be quite possible to think that the term "martyr" was an appropriate one tomuse for the suicide bombers of July 7th, and yet be wholly opposed to what they did. It depends on how you define "martyr".

I would suspect that a good number of those who answered the question that way disapproved strongly of the bombing and of the thinking that lay behind it.