Therefore I will be unto them as a lion. The verb, וָאֱהִי is the future changed into the preterit or past tense by vav consecutive, and marks the consequence of forgetting God. So Aben Ezra: "The preterit in reference to the evils which Jehovah brought upon them."
While the past thus implies that the punishment has commenced, the futures which follow denote its continuance. Rosenmüller regards the preterit hero as prophetic and continuative, and paraphrases the meaning by, "I have at length become and have been, and shall continue to be to them."
He considers the reference of the preterit to be to past disasters, especially the various defeats sustained by Israel at the hand of the Syrians (2 Kings 8:12; 2 Kings 10:32) and the Assyrians (2 Kings 15:29).
He also very aptly compares Isaiah 63:7-10 in relation to the subject in hand. The Prophet Isaiah, after relating the loving-kindnesses of the Lord and his praises and his great goodness to the house of Israel on the one hand, and their rebellion and vexing his Holy Spirit on the other hand, adds, "Therefore he was turned to be their enemy, and he fought against them."
As a leopard by the way will I observe them. The lion and the leopard are frequently conjoined, as animals of like natural ferocity, by the ancients both in sacred and secular writings. The outlook on the way is for the purpose of springing upon the passers-by.