Even a thought exhibiting the contrary fruit is sin. Even if we are in the habit of taking our thoughts captive we still have bad thoughts.
Temptation is not sin.
Paul wrote...
1Co 10:13 There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
There is ALWAYS a means to escape sin.
James wrote this...
Jas 1:12 Blessed is the man that endureth temptation: for when he is tried, he shall receive the crown of life, which the Lord hath promised to them that love him.
Jas 1:13 Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man:
Jas 1:14 But every man is tempted, when he is drawn away of his own lust, and enticed.
Jas 1:15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
Sin unto death involves an act of the will. A "wrong thought" is not a sin unto death, it can lead into a sin unto death if it is not put to death via the Spirit of life in Jesus Christ.
There is sin unto death and sin not unto death. Sin unto death is rebellion and that has ceased in the life of a genuine Christian. Sins not unto death are not rooted in a wicked heart but rather immaturity and ignorance. It is important to make the distinction. Many do not want to make the distinction because they wish to blend rebellion in with mistakes and believe in a provision which cloaks it all.