I think the 2012 report was very clear on his inexperience being a big factor and the question of why he was replaced at short notice was raised. I read something a few daya ago about the experienced match commander (Meggs?) being replaced because of a disagreement with the chief constable (Wright?). Wright was the one behind the cover-up.
In the current inquest Duckenfield admitted that he wasn't as thorough as he should have been in familiarising himself with the stadium and safety plans, so it does seem that his failure to prepare diligently was a factor in the mistakes he made on the day. But he shouldn't have been put in that position as he had no prior knowledge of the ground.
The problem was that he did something. An officer outside the ground reported over crowding outside and asked Duckenfield if he could open gate C (the large exit gate). Duckenfield said yes, because he wanted to prevent injury. However, he didn't order the closing of the tunnel into the central pens, which he should have done, and this led to the crush and the deaths. This was probably due to his inexperience, but his lack of preparation was also a factor. In this sense the deaths were an accident, with him being culpable to some extent.
He quickly realised the mistake and started the story that drunk Liverpool fans forced the gate. While we can appreciate that in his panic he might have lied to cover-up, the fact he continued to lie and his seniors instigated false testimony and a publicity campaign to propagate the lie (briefing the press and politicians) was the real crime. Wright (?) was the one most responsible here, but the whole senior command of South Yorkshire Police must have known. The coverup should have seen people in prison for perverting the cause of justice and other crimes, but this is a separate issue from causing the deaths.
In the recent inquest they got him to admit that it was his mistakes caused the deaths.
P.S. The families did bring a private prosecution against Duckenfield that was unsuccessful. The judge ordered a stay against further prosecutions against him. This could be revoked, but there are issues of double jeopardy in bringing a new trial.