What is utility? A modern interpretation is mathematical: it is that which is maximized (subject to constraints). This is quite similar to the modern mathematical approach that believes in 'meaning is use' (a la Wittgenstein): a straight line is that which is the shortest distance between two points etc. In the same vein, rational behavior is that which is the outcome of having maximized utilities.
From this perspective, suicide attackers for example, are fully rational. A lot of criticism of economics centered on the so-called 'failure to account for irrationality' feature, simply misses the point.
