In his ways - Rather "in his double ways" (as in Proverbs 28:6). The evil of vacillation rather than that of craft, the want of the one guiding principle of right, is contrasted with the straightforwardness of the man that "walketh uprightly."
Shall fall at once - Better, shall fall in one of them (his ways). The attempt to combine incompatibilities is sure to fail. Men cannot serve God and Mammon.

Shall fall at once - Shall fall without resource, altogether.

Whoso walketh uprightly shall be saved, Or "be safe" (r) from those that seek his life, plot against him, shoot at him, as the wicked do at the upright in heart, but the Lord protects him; and it is even well with him in times of public calamities; the Lord has his chambers and hiding places for him; and he is safe from falling, as may be gathered from the opposite clause; for he walks surely, and is in the hands of Christ, and is kept by him from a final and total falling away: and he shall be saved also with an everlasting salvation; from sin, and all the effects of it; from the curse of the law, from wrath to come, from hell and damnation. Not that his upright walk is the cause of this; the moving cause of salvation is the grace of God; the procuring cause, our Lord Jesus Christ, the only Author of it: but this is a descriptive character of the persons that are and shall be saved; it is a clear case that such have the grace of God, and therefore shall have glory; See Gill on Proverbs 10:9;
but he that is perverse in his ways; "in his two ways", as in Proverbs 27:6; or many ways, and all perverse and wicked:
shall fall at once; his destruction shall come suddenly upon him, when he is not aware of it, and when he cries, Peace, peace, to himself: or in one of them; in one or other of his perverse ways.
(r) "erit salvus", Pagninus, Montanus, V. L. Mercerus, Cocceius, Gejerus.

Uprightness will give men holy security in the worst times; but the false and dishonest are never safe.

(Compare Proverbs 10:9; Proverbs 17:20). Double dealing is eventually fatal.

18 He who walketh blamelessly is helped,
And he who is perverse in a double way suddenly perisheth.
The lxx translate תמים by δικαίως (as the accusative of manner), Aquila and Theodotion by τέλειος; but it may also be translated τέλειον or τελειότητα, as the object accus. of Proverbs 2:7. Instead of עקּשׁ דּרכים, Proverbs 28:6, there is here נעקּשׁ דּרכים, obliquely directed in a double way, or reflex bending himself. At Proverbs 28:6 we have interpreted the dual דּרכים rightly, thus בּאחת cannot refer back to one of these two ways; besides, דּרך as fem. is an anomaly, if not a solecism. בּאחת signifies, like the Aram. כּחרא, either all at once (for which the Mish. כּאחת, Aram. כּחרא), or once (= בּפּעם אחת), and it signifies in the passage before us, not: once, aliquando, as Nolde, with Flacius, explains, but: all at once, i.e., as Geier explains: penitus, sic ut pluribus casibus porro non sit opus. Schultens compares:
"Procubuit moriens et humum semel ore momordit."
(Note: Aeneid, xi. 418.)
Rightly Fleischer: repente totus concidet.

At once - Once for all; so he shall never rise more.