I was thinking about that last night. Your immediate response is that yes Mr Wilcox is to blame for Leonard's death (and various other things, societal wise - spoilt by muddled thinking etc). Yet if you look closely at how the plot develops it is entirely the fault of the Schlegels that Leonard ever ended up in that house, being attacked by Charles.
Some phrases echoed in my head, Have I done wrong and I've done wrong uttered by both Henry and Leonard, AND both Helen and Margaret admit they have done wrong too, in so many ways (Helen says she effectively seduced Leonard, and did not care for him, and Margaret admits she has spoilt Henry by acquiesing to his muddle headed thinking previously) There is this sense that everyone has done wrong, the house, Howard's End is the only thing that is above the muddle, and human relations, connection, sympathy itself are above the muddle.
Henry is very fond of his children, he tries to advise them, but they misunderstand him, misinterpret him. He tells Charles to go and sort out the Howard's End situation but not to do any violence. It would have been better if he had not advised anything. Advice and suggestions are constantly shown as treacherous. Kindness in the sense of understanding people and putting up with their foibles is not shown as a particularily successful method of education.
When Margaret bars the door (gate) to Henry and tells him to go away and leave her and Helen alone, for the first time she is being unkind and firm, there is no persuasion or language really just My Darling forgive me [to Helen]
And the bits about loneliness...wow.