Differential drive robot doesn't move as programmed ?

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Hi,

I was trying to create a Wall following differential drive robot based on the ranges data obtained from 2D Laser scan sensor by following this tutorial given in this Link(Part 7). I had custom modified the Python code(given below) for the node unlike given in the tutorial. It seems fine to me but when I run it using rosrun it moves in a unsual manner as shown in this video (which I have uploaded) even if I set the angular velocities to be zero. I don't know what is the problem ?

I am using ROS noetic in Linux Mint Cinnamon 20.3 and version of Gazebo is 11.10.2.

The Python Code:

#! /usr/bin/env python3

import rospy
from sensor_msgs.msg import LaserScan
from geometry_msgs.msg import Twist

dist = [0,0,0]
state_ = 0
def clbk_laser(msg):
    dist[0] = msg.ranges[0] #LEFT
    dist[1] = msg.ranges[359] #STRAIGHT
    dist[2] = msg.ranges[719] #RIGHT
    print(dist)
    move_accord(dist)

def move_accord(dist) :
    d = 0.1
    if dist[0] < d and dist[2] > d and dist[1] < d :
        state_ = 2
    if dist[0] < d and dist[2] > d and dist[1] > d :
        state_ = 0
    if dist[0] > d and dist[2] < d and dist[1] < d :
        state_ = 1
    if dist[0] > d and dist[2] < d and dist[1] > d :
        state_ = 0
    if dist[0] > d and dist[2] > d and dist[1] < d :
        state_ = 2
    if dist[0] > d and dist[2] > d and dist[1] > d :
        state_ = 0
    change_state(state_)    

def change_state(state_):        
    msg = Twist()
    if state_ == 0 :
        print("Straight")
        print(state_)
        msg.linear.x = 100
        msg.angular.z = 0
        pub.publish(msg)
    if state_ == 1 :
        print("Turn Left")
        print(state_)
        msg.linear.x = 0
        msg.angular.z = 0
        pub.publish(msg)
    if state_ == 2 :
        print("Turn Right")
        print(state_)
        msg.linear.x = 0
        msg.angular.z = 0
        pub.publish(msg)

def main():

    global pub,dist,state_
    rospy.init_node('laser_scan_test')
    pub = rospy.Publisher('/cmd_vel', Twist, queue_size=1)
    sub = rospy.Subscriber('/two_wheele_robo/laser/scan', LaserScan, clbk_laser)
    rate = rospy.Rate(1)
    rate.sleep()
    rospy.spin()

if __name__ == '__main__' :
    main()