But isn't it worse in winter?
They could have been ski-mountaineering too. Climb up, ski down.
Smaller peaks like Hood & Rainier are doable year-round. Big ones, Denali, Everest, K2, etc only have a small window during the year when they are climbable. Usually May to early June.
"But isn't it worse in winter?"
Winter is dry season in some areas (Himilaya).
Snow and ice "cement" otherwise loose rock to the mountain.
Etc.
Not necessarily. In the winter, the snow is fresh and ice is frozen solid. In warmer conditions, snow can "rot" - get holey and unstable and ice can melt, forming dangerous crevasses.
The climbing season on Mt. McKinley lasts from late April or early May until the snow gets too soft, in late June or early July.
Yes, it is, because of the much lower temperatures and the unpredictable white-out storms with 140 mph winds. Numerous climbers and rescuers have been on TV saying so. Yes, it can be done, but should it be is an altogether different issue. It's a matter of wisdom, judgment and consideration for your loved ones. Now there is at least on bereaved set of parents, a widow, and some fatherless kid(s), all so that some guy could massage his ego by climbing this peak in the dead of Winter. I'm sorry he's dead, but my sympathy is mainly with his family.