Why should they turn it down. The bequest is left in the name of his wife who, one assumes, had links to the organisation.
Even if it wasnt, can charitable organisations afford to turn down significant donations - I think not.
And it is not as if he is still around to benefit in any way from an association with the organisation.
It is in the name of his wife, maybe she asked for it to be donated.
He could be truely repentan of his crimes.
Top and bottom of it, the Guides are in the same predicament as the allies at the end of the war, when a huge slab of medical information was given to them. The info had been obtained by torturing Jews and other prisoners (you know, like dunking people in ice water until they died to see how long it took).
The medical info is/was not evil, merely the method of its gathering. Likewise this Â£400,000 is not evil. Merely he donating person. Perhaps it could be put in to bringing abused girls in to the guides, or educating the guides about such action.
Accepting the money does not mean the Guides change their motto to "we love nonces".
I can see both sides of the argument here and I would not like to be in the GG's shoes on this one. There is no doubt that Â£4,000,k would be of benefit to them and to their members but there are some who would feel that the money is tainted and that would taint whatever it is used for.
Perhaps the best thing they could do is take the money and quietly pass it on to a sister organisation in a less well off part of the world who would not have to know where it came from.