Witnessing and warning is one thing. Gloom and doom is quite another, and can drive people away in droves.
I mean sure, there are other people who are attracted to gloom and doom. You can draw them. That's why almost every town diner has the old men's table where the guys sit around with coffee and complain about everything all morning.
But in general it won't help reach the lost.
People want the truth, they have to make a decision for the Lord and it is not fair if they are not given all the facts and all the truth.
The truth is that the is world has been condemned. That when the judgement comes everything that you built with wood, hay and stubble will go up in flames and you will suffer loss. The only thing that is eternal is gold (the pure word of God), silver (the Lord's redeeming work on the cross) and precious stones (testimonies of being transformed by the Spirit working within you).
The best coach I ever had was very up front and honest about our opponents. He scouted them and he prepared us for them.
If you don't talk about what they will have to face how can they possibly be ready for it. Watchman Nee said a martyr has the attitude and mindset of a martyr long before they get martyred. Paul said he had been crucified with Christ and it was no longer he that lived long before he was martyred.
The worst coach I had told me I would be the starting goalie in our championship game right before the game. He could have told me two days earlier and I would have been better prepared mentally.
I don't feel any gloom and doom, Jesus Christ has already won the victory. But every athlete knows they will prepare for the competition mentally and physically. You can feel the adrenalin running through you. Before every swimming match I would have to use the bathroom. Our football team had a curfew the night before to make sure we had enough sleep.
Jesus was out praying the night before the crucifixion and told the disciples repeatedly to watch and pray with Him. If they had perhaps Peter wouldn't have denied Him, maybe they wouldn't have struck the ear of one of the soldiers with a sword, maybe the one guy would not have run away naked.