The beginning of the parsha describes how, after Bnei Yisrael were chased out of Egypt, the Egyptians had a change of heart and they set out in hot pursuit of their former slaves. The Torah (Shemos 14, 7) tells us that Paraoh took, "Six hundred elite chariots and all the [rest of the] chariots of Egypt" and chased after Bnei Yisrael.
The question is from where did the Egyptians obtain the horses for these chariots? They could not have been their own animals since they died in the plague of pestilence and the plague of hail. They also couldn't have been Jewish owned animals because Bnei Yisrael took all their livestock along with them. Rashi, citing the Medrash, answers that they belonged to the G-d fearing Egyptians who had gathered their animals into their houses when they were warned about the pending plague of hail. These animals survived the hail, and it was these animals that were mobilized to chase after Bnei Yisrael. Regarding this incident Rebbi Shimon commented, "[Even] the upright among the Egyptians - kill him; [even] the best snake - smash its head."
Rav Wolbe (Shiurei Chumash, Alei Shur vol. I pg. 145) elaborates on Rebbi Shimon's observation. Despite the fact that these Egyptians had clearly demonstrated their fear of G-d, nevertheless, by nature they remained with a lack of yiras shamayim. The Egyptians are compared to a snake, since it is possible to tame a snake and train it to act docile. Yet, the Mishna tells us that a snake is always considered a mu'ad - an animal that is intrinsically dangerous.
Every person is created with positive and negative Middos. Our avodah is to rectify the negative traits lest they be manifested in our actions. However, even if we succeed in this endeavor, we haven't completely uprooted the trait. We have merely suppressed it or channeled it toward a positive outlet. Hence, we must always be on guard to ensure that these negative Middos do not rear their ugly heads at a later point.
Why must it be this way? Why can we not rid ourselves entirely from these negative traits? The answer is that bechira (free will) requires that there always be a negative force to counteract the positive. If we would have the ability to entirely uproot our negative Middos we would upset this balance and lose our bechira. Instead, the roots of these Middos remain, thereby requiring us to pay constant attention to our Middos.
If people would pay more attention, they would notice that they have specific negative Middos that surface regularly. It is those middos that are their battleground. They cannot be uprooted entirely, but if they make an effort, they can restrain these Middos or possibly even channel them to be used positively.