There are degrees in sin. There are the great sins, of which the taking of life is the most evident; and the small sins, like, for instance, lying for convenience.
Then there are the hidden sins. Pride is an obvious one. But curiously selflessness can also be another when that selflessness trumps what is right.
Of course, selflessness on the grand scale, à la Schweitzer or Mother Teresa, is admirable and, if you have the inclination, should be emulated. But selflessness can also be obscene, as with Poland's Colonel Mastalerz in the early days of the Second World War, who led his men to a futile but selfless death in a cavalry charge against modern armour at overwhelming odds in what must surely have been a act of moral cowardice, though advertised as selfless bravery.
So often, day to day, people conduct apparently selfless acts whereas in reality they do the wrong thing out of expediency either to avoid confrontation, as when indulging a child, or for the "righteous" pleasure of self-sacrifice. Doing what is right does not always mean doing what is selfless. The dollar to the beggar is a noble gesture but with the caveat that it could be used to buy whisky whereas that same dollar to the Salvation Army may be the less convenient but wiser approach. Instances of selfish selflessness abound and these can sometimes (but by no means always) be very great sins.