Market Folly has a great summary of Tilson’s rationale here. In short, it’s a case of being greedy while others are fearful. Tilson compares BP to the Texaco v Pennzoil litigation in the 80s. Tilson makes the point that shareholders in Texaco “weren’t harmed” when Texaco filed for bankruptcy protection following Joe Jamail’s $12 billion judgement against the company. Here’s Icahn describing the negotiations to settle the litigation:
Icahn made out like a bandit on his holding in Texaco. One wrinkle to this comparison is that BP is a British stock, and so it’s not subject to the same bankruptcy regime as Texaco. The Texaco matter also wasn’t as politically sensitive as the BP spill.
The Wall Street Journal has an interesting analysis discussing various scenarios for BP (subscription required).