The most famous dark site of the country – and a classic one regarding the role of popular media in making dark tourist destinations major attractions – is the Bridge on the River Kwai. Yes, it really exists ... though it looks very different to the fictional bridge in the famous film. And it has become a major sight for mainstream tourism and dark tourism alike. The reason it's dark is that it forms part of the so-called Death Railway, a rail line built by the Japanese during WWII. More precisely: the Japanese had it built, largely by POWs, including many Europeans and other Westerners (most notably Australians), under an incredibly brutal regime that claimed countless lives. There are also several memorial sites/museums on the topic, primarily in the town of Kanchanaburi, where the famous bridge is, but also further afield (at Hellfire Pass).
The capital Bangkok also has its dark sites in between the usual mass tourism and nightlife that the city is much better known for. One is a rather grim prison museum, the other a monument and documentation centre dedicated to one of the uprisings that shook the country, in 1973, near the better-known Democracy Monument, which is regularly the rallying point when there's political trouble in the country.
Disruption still comes periodically in the form of political upheaval, which can directly affect tourism too, e.g. during the siege at the international airport leaving thousands and thousands of travellers stranded in 2008. A couple of years later even civil-war like violent clashes broke out right in the city centre. And again mass protests against the government mounted in late 2013.
However, the most disastrous blow to the country's tourism industry came in the form of the catastrophic tsunami of Christmas 2004. Much of the destruction has been cleared up and rebuilding has resurrected much of the devastated coastline infrastructure so crucial to tourism. But it's probably still not quite back to where it was before the disaster.