I've actually wondered for a while on this point- when it comes to a deathcurse, does the wizard even need to bother with controlling it, or can he blow his entire load of consequences and FP on raw power?
After all, he can just opt to take fallout for a failed control check, and since 1) he's about to be taken out anyway, taking more fallout than he can handle is no big... and 2) since 80 physical stress doesn't kill me any worse than 8 (you don't care either way, being dead and all), you may as well go for broke and forget about it?
Normally, I'd just let death curses fall under 'plot device' and not worry about it.
[Edit]: There does seem to be some established canon on some death curses being more powerful than others, based on the strength of the caster, and how much of a beating he'd taken beforehand... if backlash is unlimited, then there's no reason one deathcurse is any rougher than another, since you can also go for broke over-charging the spell way beyond reason, too.