I would speculate that he lost his rag-------that is, IF he did it at all, and I am certainly not convinced he did.
The main thing which people can’t their head around in this case is the contrast between what Mark said / says and his subsequent actions, such as fobbing off neighbours with the London Coptic Church tale but making no further enquiries himself, or the fact that within a day of returning home after a month’s absence and seeing that everything was going to plan regarding his father’s wishes for a tree root-barrier next to the garage, he should suddenly decide to interfere and order two and a half tonnes of concrete, then dump it directly over where his father was interred. All this when he freely admits to being only a pen-pusher, uninterested in DIY and isn’t used to heavy manual work.
I know what it’s like to order ready-mixed concrete – if you’ve never done it before, you’re on edge wondering if everything will go to plan, then when a bl**dy great lorry arrives whirring away, you wonder what you’ve let yourself in for! In my case it was simply chuted directly into a hole for foundations and later into a shuttered area for a floor slab. Luckily both turned out OK. Mark didn’t have that straightforward advantage – because of poor access, he had to barrow load after heavy load, negotating each through a narrow gateway, no doubt spilling much on the way. Then he had the problem of wasting more time waving traffic past the lorry which was blocking access to the cul-de-sac, when the driver is allocated only a certain period for depositing his load.
Which beggars belief why Mark should have attempted such a demanding task in the first place!?
Then of course is the awkward question why he told Senta he'd seen his father in the few days before being arrested?