Obviously my question comes with it's own bias, and how judged he is is somewhat of a hard point to prove, but a short list of things Churchill can be considered at least partially responsible for:
The Gallipoli Campaign in WWI as First Lord of the Admiralty.
The Allied policy of bombing civilian targets in WWII.
The firebombing of Dresden specifically.
The exacerbation of and then indifference by British high command to the Bengal famine.
Pressuring to put Montgomery in charge of an invasion of the Netherlands and Germany (Operation Market Garden). Which was a huge failure.
Many of these decisions were military based and not necessarily political or diplomatic, but he used his position as Prime Minister to realize them. Why is he often "let off the hook" for the most part when his legacy is being discussed, with many historians and biographers either glossing over these incidents or ignoring them entirely? Overall, it seems like in part he was a former military man with a spotty record, who as a politician interfered with military operations or didn't interfere when it came to human rights, sometimes to disastrous effect.
I think most people who take an active interest in history and most academics in history do see Churchill as a more morally ambiguous character. It's just the mass media in the UK which chooses to idolise him, much as the mass media in the US idolises George Washington and the Indian media idolises Ghandhi. They're all built into an almost mythical origins stories for the nation. Why people and governments feel the need to create this mythos is another question but it seems pretty universal (e.g. Romulus and Remus for Rome).
Specifically on Operation Market Garden, even though this was unsuccessful it has had its powerful defenders. If I remember JFC Fuller's argument correctly, he viewed the war carried out by Eisenhower as a timid one. Its success was pretty much inevitable, but slow. Market Garden was not an inevitable failure but a risk that had it come off could have ended the war much sooner.
He was widely blamed for the failures in Gallipoli. It is interesting to read contemporary sources for the inter war period such as Harold Nicholson and Chipps Channon's diaries when he was seen as a spent force.