I think that a lot depends on how you interpret the word "sympathise". It is rather too vague to draw too firm conclusions. You can sympathise to some extent, or greatly, or something between, and each carries a different meaning, and then it also depends on how the respondents interpreted the question.
It is entirely possible to sympathise with a subset of motives behind a set of horrific actions without condoning the actions themselves. One example from history might be the French Revolutionaries, cutting their bloody way through all who disagreed with them. Horrific. And then you go and visit somewhere like Versailles, see its in-your-face "I'm rich and you, my subjects, are all so poor", and begin to wonder if, after all, the revolutionaries had a point. I can sympathise, then, with some of the motives behind the revolutionaries to some extent, such as their need to overthrow oppression, without condoning the bloody executions of their many victims.
Such sympathies may be misplaced, of course, but I think it's dangerous to read into this 27% figure that one quarter of British Muslims are happy with what happened at Charlie Hebdo.
The 11% who agreed that the journalists deserved their fate, however, is disturbing. I'm slightly surprised that this figure has been overlooked.