Widowmakers are pretty much the balls of khador list making. They do what you need and do it well. As far as your question:
For the league you only have basically one static list so I would look to cover what bases I could at least to some extent. This is a much more casual league and meant to get new people like yourself into the community.
For a steamroller or other tournament you have two lists so typically you will see one of two things. Either a player will have two well-rounded casters/warlocks which can perform well in both scenario based play and have kill-box/assassination presence. Or, you can go the other route and have one heavily focused on each aspect.
A well rounded list should have something to deal with the following at the very least if you want success:
-High armor (20+)
-High def/stealth models
-Spells/anti-magic
-Weak points your caster has
-a solid objective holder
-something to tie up/kill infantry
-a way to kill a caster effectively (this means no relying on outlandish rolls)
-Khador should probably also find ways to either get mobility or hinder the opponents mobility
Assasination lists should have the following concerns, with the above covered afterwards:
-Mobility methods up the ass
-something to deliver the killing blow (usually the caster)
-ways to clear a path
-delivery system
-protection/failsafe
Scenario guys should look for this, plus a good mix of the first list:
-High armor guys or a tarpit unit
-High defense guys
-mobility
-abilities like dig in, stealth, camo, incorporeal, or some other way to stay in there.
-a good aoe to clear off an objective
-an ace in the hole in case winning by scenario aint happening.
Khador should always have a way to deal with stealth and incorporeal, as magic is scarce beyond the warcaster. I recommend the grey beards (or whatever the old dudes with ice picks are) as the have solid utility and magic, plus sprays are great.
There you go, Goss's tactica of the day.