The main evidence against him was a video shot by Graham Hall. It was the 31st successful prosecution for animal cruelty based on undercover evidence provided by Mr Hall, who for seven years has operated behind the lines in the world of badger-baiting and dogfighting.
Mr Hall met Smith in a pub after a tip-off. 'He said he was into badgers, dogfighting, cat killing, the lot.'
The link was cultivated by phone for a month. At last Mr Hall was invited along for 'weekend sport'. Pints of lager were downed before Smith turned torturer. Mr Hall shot his film and escaped with the cat, but it died 11 hours later.
Mr Hall, 36, does not fit any stereotype of a do-gooder and freely admits to a chequered past. 'When I was younger, I did a bit of burglary. I ain't proud of it.' But when he was 14, he stumbled across badger-diggers and watched, sickened, from bushes as dogs were set on the unearthed animal.
'I get satisfaction exposing these people,' he says.
'I would like to be a fly on the wall the morning they open a newspaper and see their faces. I've been with someone who's been talking about this chap Graham Hall; how he infiltrates people; the way he films people. And I've just filmed the silly bastard who was saying it. He hasn't got a clue what I look like or where I come from.'
His first inquiry was into dogfighting in Ireland. He joined a group of men at a makeshift ring in a factory outside Dublin. He clicked away at the crowd with a tiny stills camera. Four men were later jailed, but he was tracked down. Lured by a phone call to price a job - he is a painter and decorator - he was seized by four men, bundled into a van, and taken to a field where he was stripped, tarred and feathered. He escaped by stabbing one captor with his own knife and ended up in hospital being scrubbed with solvent.
Other coups include the jailing of five badger-baiters from Powys after he shot a 45-minute video of the men laughing as badgers were shot, stabbed, and torn apart by dogs. His exposure of an Irish farmer who offered weekend breaks for English badger-diggers in Co Cavan stopped the trade. The farmer was earning pounds 20,000 a year from it.
But others in the animal-welfare world are wary of Mr Hall. Partly this is because he has broken away from establishment groups such as the RSPCA and the League Against Cruel Sports and partly because of his practice - between filming the crime and eventual prosecution - of selling the evidence to newspapers.
The RSPCA believes this alerts many people, enabling them to bolt for cover. Mr Hall says he needs the money for the next investigations. In his best year he made pounds 14,000. Top money for a News of the World exclusive was pounds 3,500.
Mike Butcher, chief inspector in the RSPCA's Special Operations Unit, said: 'Our hierarchy are fighting shy of him.
'Graham's got bottle and is prepared to get his hands dirty. He is an ace infiltrator because he can think like they do. But he needs to come back into the fold.'
Such a return is unlikely. Three years ago, Mr Hall set up UK Animal Watch, which sends newsletters to its 200 members. He has applied for charity status, but meanwhile relies on street collections.
'In the past, we did all the dirty work, then handed the prosecution over,' he says. 'I gave my evidence but UK Animal Watch never got a mention in court. The RSPCA does a good job but it only works on tip-offs. They don't get out there and investigate.'
John Bryant, head of research at the League Against Cruel Sports, also praises Mr Hall - 'He looks more like a badger-digger than a proper badger-digger' - but warns that such operators can become hooked on the 'sport' itself. 'An animal rightist wouldn't be able to stand by and watch these bloody awful things.'
Mr Hall is adamant that he can switch off from the horror. Behind a camera, he's too busy for emotion to creep in.
But there are moments of savagery he cannot banish from memory. On a castle estate near Middlesbrough, he joined poachers using lurchers to fell a running deer. 'Then a bloke runs up, yanks the head back and saws at its throat with a breadknife. I have never forgotten that sound, the gargling of the blood. They took it back to a kitchen, skinned it and chopped it up for the freezer.'
Death threats are common. He was once sent a video showing a man hacking off a dead badger's head with the caption: 'You're next]'
'Someone's going to have me one day - I've come very close,' he says. 'But when I listen to a tape-recording of a man telling how he hangs live cats and ferrets from beams face to face, I'm prepared to put my life on the line.'